In block:

,

,

Conversion: display:inline,block,inline-block.

Difference:

block,

①Always starts on a new line and occupies a whole line;

②Height, line height, margins and inner margins The distance can be controlled;

③The bandwidth is always the same as the browser width, regardless of the content;

④It can accommodate inline elements and other block elements.

Line,

① and other elements are on the same line;

②Height, line height and outer and inner margins can be changed;

      ③ The width is only related to the content;

      ④ Inline elements can only accommodate text or other inline elements.

Group query：orderby

The number of men and women in the table is 20-30：

select count(sex) from Table where age between 20 and 30 group bysex

ORM：

Object Relational Mapping (ORM) mode is a Technology to solve the mismatch between object-oriented and relational databases

LinuxView port: Brush selected file: View process:

        netstat-antu      // View all current tcp ports ·

##           netstat-ntulp | grep80     //                                                                                                                                                                                        . Situation·

Linux

Check how many commands are executed: historyFile operation function:

open Open the file

fstat Get the file

file Read the entire file into an array

fclose Close the file

fwrite Write to the file (safe to use Binary file)

file_get_contents() reads the entire file into a string

Several ways to open a file:

Fopen(), file_get_contents ()

Read and write, append

php

Delete folder command Unlink();

Chinese string interception:

mb_substr

mb_substr($str,$start, $length, $encoding)

$str, the string that needs to be truncated

        $start, the starting point of truncation, the starting point is 0

                      $length,                       $encoding,                                                                                                                                                                                    The difference between

#D

and M

:

M instantiates the base class, and D can instantiate the custom class table

Interlaced color change:

Use JQ to identify the even and odd numbers of table tr td, and assign different CSS styles to the corresponding odd and even numbers. Through different backgrounds, you can achieve interlaced color change. Effect.

When the mouse passes by, JQ calls the trtd background set separately, so that the background color changes when the mouse passes by.

ajax Synchronous and asynchronous:

Synchronously wait for the return result from the server and then perform other operations

After sending the request asynchronously, perform other operations directly without waiting for the return result from the server

The default is: asynchronous

Linux View all files: ls

Set a p on another p:

The larger the z-index value, the higher it goes.

OB function:

# OB_START (); // Open an output buffer, all output information will no longer be sent directly to the involved , but stored in the output buffer.

Ob_clean(); //Delete the contents of the internal buffer without closing the buffer (no output).

Ob_end_clean(); //Delete the contents of the internal buffer and close the buffer (no output).

Ob_get_clean(); //Return the contents of the internal buffer and close the buffer. Equivalent to executing ob_get_contents() and ob_end_clean()

    ob_flush();    //Send the contents of the internal buffer to the browser, delete the contents of the buffer, and do not close the buffer.

Ob_end_flush(); //Send the contents of the internal buffer to the browser, delete the contents of the buffer, and close the buffer.

Ob_get_flush(); //Return the contents of the internal buffer, close the buffer, and then release the contents of the buffer. Equivalent to ob_end_flush() and returns the buffer contents.

flush(); // Output the content released by ob_flush and the content not in the PHP buffer to the browser; refresh the content of the internal buffer and output it.

Ob_get_contents(); //Return the contents of the buffer without output.

Ob_get_length(); //Returns the length of the internal buffer. If the buffer is not activated, this function returns FALSE.

              ob_get_level();             //Return the nestinglevel of the output buffering mechanism. ##3.

How to make a shopping cart? process. .

1. Add the product to the shopping cart

1. First add a link on the page 1. Get the product id first

2. Query the product by ID from the database.

3. Add the product to the shopping cart

a. Get the shopping cart from the session, if not, create a new one

b. Determine whether the product is in the shopping cart Does it exist? If not, the quantity is 1. If it exists, the quantity is increased by 1

c. Reload the shopping cart into the session.

Problem: Our shopping uses a HashMap collection, the key is unique, and the only way to ensure the key is through the hashCode and equals methods

So we are using When rewriting, we need to rewrite the hashCode and equals methods of the Product class. When we rewrite, we only need to compare the id values.

2. Display shopping cart items

Our shopping cart exists in the session. We only need to collect the Cartt Map in the session on the cart.jsp page and display it.

1. Manipulate the items in the shopping cart

2. You can modify the quantity of items in the shopping cart when clicking or -.

3. When the or-button is clicked, we will call a function in javascript. changeCount(); Through this function, we send a request to the server, obtain the data in the shopping cart from the session on the server side, and modify the specified number of items in the shopping cart based on the submitted data before returning to the shopping cart page for display.

#How to get the sales volume of the last month? Three months? What about a year?

                                                Check a certain field in the database

How to retrieve 30 million pieces of data within 3 seconds? What index to create?

Ordinary index

Index type:

Ordinary index, unique index, primary key index, combined index

# The difference between

##redis and memecache is:

1. Storage method:

Memecache stores all the data in the memory. After a power outage, It will hang because the data cannot exceed the memory size. Part of redis is stored on the hard disk, which ensures data persistence.

2. Data support type:

Redis has much more data support than memecache.

3. The underlying model is different:

The new version of redis directly builds the VM mechanism by itself, because if the general system calls system functions, it will waste a certain amount of time to move and request.

4. Different operating environments:

Currently redis officially only supports LINUX, thus eliminating the need for support for other systems. In this way, you can better devote your energy to this system environment Optimization, although later a team from Microsoft wrote a patch for it. But it is not placed on the trunk

SKUHow to do it:

Attribute specifications.

In fact, sku is the product inventory unit, including the product table, product attribute table, product table

tp configuration file: common/config.php

php data type:

Four scalar types:

boolean (Boolean )

integer (integer type)

float (floating point type, also called double)

string (string)

Two composite types :

Array (number of arrays)

Object (object)

Last two special types:

Resource (resources)

# NULL (NULL)

Singleton mode:

The main feature of the singleton mode is "three private and one public":

A saving class is required The private static member variable of the only instance of Being cloned

must provide a public static method (usually named getInstance) to access this instance, thereby returning a reference to the unique instance.

Query the number of men and women between 20-30

:

//select count(sex) from table where age between 20 and 30group by sex

Select count(*) from table name where age between 20 and 30 group by age. Group by

PHP

There are three access modifiers, namely:

public (public, default) protected (protected)

private (private)

Access permission modification Scope of operators:

When a member of a class is declared with a public access modifier, the member can be accessed and operated by external code.

private (private)

Members defined as private are visible to all members within the class and have no access restrictions. Access is not allowed outside the class.

protected (protected)

protected is a little more complicated. Members declared as protected only allow access by subclasses of this class.

mysql intStorage:

bigint The storage size is 8 bytes.

The storage size of int is 4 bytes.

The storage size of smallint is 2 bytes.

tinyint Integer data from 0 to 255. Storage size is 1 byte. The scope of

cookie:

domain itself. All domain names under domain.

It is to set a permission for the cookie. When domain is set to empty, domain defaults to the current domain name, and subdomains under this domain name can receive cookies.

But when the domain parameter sets its subdomain name, all domain names cannot be received, including that subdomain name.

linuxCommands to view log file contents

tail, cat, tac, head, echo

tail-f test. LOG

Top View memory

# DF-LH View Disk

PS-A View all processes

## constant:

Use $ for variables:

There is no need to use the $ symbol when defining and using constants.

The value of a constant must be a fixed value and cannot be a variable, class attribute, the result of a mathematical operation or a function call.

Constants can also be defined in interfaces.

Abstract classes cannot be instantiated. Abstract classes can have static methods. There can be no abstract methods in abstract classes.

Ordinary methods are stored in the class, and there is only one copy.

Static methods are also stored in the class, and there is only one copy.

The difference is:

                                                                                                                                                                                          out Out Out Out Out Out Out Out     Out       out , , exp ’ out back out out out out out out out out out ''' over off' field field field field field field field field field field field field field field field receives itself itself then being being being being being called then being being being called the same used you you you? $this needs to be bound, that is, you can call it without an object

Static members: they can be accessed without instantiation. The static member variables of the class only belong to this class

Class members: one One is defined in the class, and the other is defined in the constructor.

The difference between methods and functions:

Functions exist alone, while methods depend on the class and can only be called through objects.

Whether functions can be defined in class files: No

Value transfer and value reference:

(1) By value Passing: Any changes to the value within the function scope will be ignored outside the function

​​​​​(2) Passing by reference: Any changes to the value within the function scope will also reflect these modifications outside the function

(3) advantages and disadvantages:

A: When passed by value, PHP must copy the value. Especially for large strings and objects, this can be a costly operation.

B. Passing by reference does not require copying the value, which is very beneficial to performance improvement.

Prevent orders from being submitted repeatedly:

1. Use JS to disable the button after clicking it once. Using this method can prevent multiple clicks from happening

2. Set the session value. After submitting it once, delete it. If there is no session value, do not allow submission.

3cookies are the same

String functions:

strlen();strpos();echo();implode();trim(); etc.,,

String reversal function:

strrev, mb_string;

The difference between http1.0

and http1.1: HTTP1.0 stipulates that the browser only maintains a short-term connection with the server. Each request of the browser needs to establish a TCP connection with the server. The server immediately disconnects the TCP connection after completing the request processing.

The server does not keep track of each client nor log past requests.

HTTP1.1 supports persistent connections. Multiple HTTP requests and responses can be transmitted on a TCP connection, reducing the consumption and delay of establishing and closing connections.

HTTP1.1 also allows the client to make the next request without waiting for the result of the previous request to be returned, but the server must send back the response results in the order in which the client request is received,

             To ensure that the client can distinguish the response content of each request, which also significantly reduces the time required for the entire download process

           HTTP1.0 does not support the Host request header field.

H The Host request header field was added to TTP1.1.

The difference between global variables and local variables:

1. Different scopes: The scope of global variables is the entire program, while the scope of local variables is the current function Or loop, etc.

2. Memory storage methods are different: global variables are stored in the global data area, and local variables are stored in the stack area

3. Lifecycles are different: the lifetime of global variables and the main The same as the program, it will be destroyed when the program is destroyed. Local variables are inside the function or inside the loop and will no longer exist when the function exits or the loop exits.

4. The usage is different: global variables are used in each part of the program after they are declared Parts can be used, but local variables can only be used locally. Within the function, local variables will be used first before global variables

One thing to note is that local variables cannot be assigned the value of a global variable with the same name.

Front-end optimization:

First: Content-oriented optimization

1. Reduce HTTP requests

2. Reduce DNS Search

3. Avoid redirects

4. Use Ajax caching

5. Lazy load components

6. Preload components

7. Reduce the number of DOM elements

8. Split components into multiple domains

9. Minimize the number of iframes

10. Don’t get http 404 errors

Second: Server-oriented

1. Reduce cookies

2. Use domain name-independent methods for Web components

What is cache avalanche ?

When the cache server is restarted or a large number of caches fail in a certain period of time, it will also put a lot of pressure on the back-end system (such as DB) when it fails.

How to avoid it?

1: After the cache expires, control the number of threads that read the database and write the cache through locking or queuing. For example, only one thread is allowed to query data and write cache for a certain key, while other threads wait.

2: Set different expiration times for different keys to make the cache invalidation time points as even as possible.

3: Make a second-level cache, A1 is the original cache, A2 is the copy cache, when A1 fails, you can access A2, the cache expiration time of A1 is set to short-term, and A2 is set to long-term (this point is supplementary)

tp Routing mode:

1. Normal mode 2. Rewrite mode 3. Compatibility mode

niginxHow to configure the port number:

Configure two servers in that nginx.conf, respectively The port number.

jqGet the second row of the table tag:

$(tr:eq(1))

What is a variable variable:

Variable variables allow us to dynamically change the name of a variable. Use the value of one variable as the name of another variable.

Check whether the port is occupied: Modify permissions Modify owner Modify user group

netstat –apn chmod chown chgrp

View process

Top pa aux|grep

Add execution permissions for everyone:

chmod-x

MysqlMaster-slave replication:

Generate two threads from the library, one I/O thread and one SQL thread;

i/ The o thread requests the binlog of the main library and writes the obtained binlog log to the relay log (relay log) file;

The main library will generate a log dump thread to provide i/o to the slave library The thread passes the binlog;

The SQL thread will read the log in the relay log file and parse it into specific operations to achieve consistent master-slave operations and consistent data;

Commands required for master-slave replication:

showmaster status;

startsalve;

stopsalve;

flushprivileges;

GRANTREPLICATIONSLAVE,RELOAD,SUPER ON *.*

TOmysql_backup@'%'

IDENTIFIEDBY '123456';

                CHANGEMASTER TOmaster_host = '127.0.0.1',

##             master_user='mysql_backup',

##                                               ’s     off ’s  ‐‐ out‐over‐to‐under‐taken to        .000001',master_log_pos = 3696;

Resolve the problem of primary key conflict

Create a master table, and the id is distributed from the master table.

Master-master replication:

1. Authorize accounts on two machines respectively: grant replication slave, file, select on *.* to'repl'@'10.17.%'identified by 'xxxx' 2. Configuration file/etc/my.cnf, when using the main library configuration file Basically, add the following configuration items

##nginx

Reverse proxy:

(1 ) rr polling (default) It is assigned to each RS according to the request order. It is the same as the rr algorithm in lvs. If the RS is down, it will be automatically eliminated. By default, only port 80 is detected. If the RS reports 402, 403, 503, and 504 errors will be returned directly to the client.

(2) Weight(weight)

Add weight (default is rr weight) on the basis of rr. Weight polling is proportional to access. The larger the value, the more allocated , the weight can be set according to the server configuration, which can solve the problem of uneven server performance and request allocation

(3) ip_hash

Solving the problem of dynamic web page session sharing

Each Access requests are allocated according to the hash value of the IP address. As long as the hash value of the IP is the same, it will be allocated to the same server (-p parameter of lvs load balancing, persistence_timeout50 in the keepalived configuration). This scheduling algorithm can solve dynamic web page sessions. Sharing problem, but sometimes it will lead to uneven distribution of requests.

Tips: Since the nat mode is used in China, hash is not suitable for use

ip_hash cannot be used with other algorithms, that is Cannot make weight or backup

(4) fair (third party)

It is configured according to the response time of the back-end server. The one with the short response time is allocated first, which is more intelligent than the above. This This algorithm can intelligently perform load balancing according to page size and loading time. nginx itself does not support fair. You need to download nginx’s upstrea_fair module

(5) url_hash (third party)

Main applications On the cache server

Distribute requests according to the accessed URL, so that the same URL is directed to the same server. The effect is more significant when the back-end server is a cache server. Add a hash statement to the upstream and server statement. Other parameters such as weight cannot be written. hash_method is the hash algorithm used.

Disadvantages: If a machine is down, it will be difficult. Consistent_hash can solve this problem

It can improve the efficiency of the back-end cache server. nginx itself does not support url_hash and needs to be downloaded hash software

(6)least_conn

Minimum number of connections, whichever device has the fewest connections will be assigned to which device

(7) consistent_hash

Consistency algorithm

3, Load balancing:

Load balancing deployment method:

Routing mode (recommended)

Bridge mode

Service direct return mode

Round-robin balancing:

Each request from the network is assigned to the internal server in turn Server, go from 1 to N and then start again. This balancing algorithm is suitable for situations where all servers in the server group have the same hardware and software configuration and the average service requests are relatively balanced.

Weight round-robin balancing:

According to the different processing capabilities of the server, different weights are assigned to each server so that it can accept services with the corresponding number of weights. ask. For example: the weight of server A is designed to be 1, the weight of B is 3, and the weight of C is 6, then servers A, B, and C will receive 10%, 30%, and 60% of service requests respectively. This balancing algorithm ensures that high-performance servers receive more utilization and prevents low-performance servers from being overloaded.

Random balancing:

Randomly distribute requests from the network to multiple internal servers.

Weight random balancing:

This balancing algorithm is similar to the weighted round-robin algorithm, but it is a random selection process when processing request sharing

4 The difference between load balancing and reverse proxy

Reverse proxy is a method to achieve load balancing.

Let’s talk about reverse proxy first. When the user makes a request, he first sends the request to the proxy server, and then the proxy server requests the real server according to the algorithm, and finally returns it to the user. This approach, firstly, improves security; secondly, it shares user requests through multiple real servers and achieves load balancing.

Let’s talk about load balancing. The emergence of load balancing is to reduce the pressure on a single server as much as possible through horizontal expansion. Common WEB-level load balancing solutions include hardware F5, Nginx proxy, LVS, load balancing services of various cloud providers (such as AWS ELB service), etc. What is connected behind the load balancing is usually the server that actually provides the service. For example, through the ELB service, the traffic can be evenly shared, thereby reducing the pressure on the stand-alone server.

Since the load balancing layer has been added, simply using a certain solution still has to consider a single point of issue. The server responsible for load balancing failed to withstand the pressure, went down, and the service was unavailable. Therefore, Nginx and LVS try to configure multiple proxies to enable failover and fault alarm, so as to deal with proxy layer server problems in a timely manner. ELB is a service provided by Amazon. Its implementation has hundreds or even thousands of machines at the bottom, so just think of it as a proxy cluster.

         

   

How to achieve session

sharing:                                                                                                                                                                                         , assuming your site is stored on one machine, then this problem does not exist, because the session data is on this machine, but what if you use load balancing to distribute requests to different machines? At this time, there is no problem with the session ID in the client. However, assuming that the user's two requests go to two different machines, and its session data may exist in one of the machines, there will be a situation where the session data cannot be obtained. So session sharing becomes a problem

1. Session sharing based on NFS

2. Session sharing based on database

3. Session sharing based on Cookie

4. Session sharing based on cache (Memcache)

5. Session copy

Scope of variables:

Global variables, local variables.

Create compressed files:

zip, gzip, bzip2, tar Zcvf;

varchar20

How many Chinese characters can be stored? First, you must determine the mysql version

4.0 Below, Varchar (20) refers to 20 bytes. If you store UTF8 Chinese characters, you can only save 16 (3 bytes of each Chinese character)

5.0 or more, Varchar (Varchar (Varchar (Varchar (Varchar (Varchar ( 20), refers to 20 characters. Whether it is numbers, letters or UTF8 Chinese characters (each Chinese character is 3 bytes), 20

can be stored. In fact, the best way is to create it in your own database. Let’s try how many Chinese characters can be put in a table. MySQL 5.0 is now available. Varchar(20) can store 20 Chinese characters.

Username and password when logging in How to store:

Generally, we use cookies to store user information on the client, such as a seven-day login-free system.

We first set the cookie parameters when logging in and store the user name and password. Next time we jump to the next page,

We can first determine whether there is a cookie If there is a value, it will jump directly to the next page. If there is no value, it will prompt you to log in.

After implementing cookie storage, users can log in successfully without entering a password!

Command to view the firewall (before Centos7.0):

View the firewall status ： service iptables status

Turn on the firewall: service iptables start

Turn off the firewall: service iptables stop

Clustered index and non-clustered index:

1. Clustered index

a) An index item directly corresponds to the storage page of the actual data record, which can be said to be "direct"

                                                                                                                  #                                                                                         #                                                                                             # If there is a primary key, find another field or a group of fields that can also meet the requirements of the primary key, and rebuild the primary key)

d) A table can only have one clustered index (reason: once the data is stored, there can only be one order) )

2

, non-clustered index a) It cannot be "directly accessed" and may access multiple objects in a chain The data page can be located only after leveling the page table

b) A table can have multiple non-clustered indexes

Click to log in:

1. Description of the login principle

The technical implementation mechanism of single sign-on: When the user accesses application system 1 for the first time, because he has not logged in yet, he will be Guide to the authentication system to log in; based on the login information provided by the user, the authentication system performs identity verification. If the verification is passed, an authentication credential - ticket should be returned to the user; when the user accesses other applications, this will be Bring the ticket as a credential for its own authentication. After receiving the request, the application system will send the ticket to the authentication system for verification to check the validity of the ticket. If the verification is passed, the user can access application system 2 and application system 3 without logging in again.

It can be seen that to implement SSO, the following main functions are required:

a) All application systems share an identity authentication system;

b) All application systems can identify and extract ticket information;

c) The application system can identify users who have logged in, and can automatically determine whether the current user has logged in, thereby completing the single sign-on function

Based on the above basic principles, I designed a single sign-on system program using PHP language, and it has been officially put into operation to generate the server. This system program uses the ticket information with the unique session id of the entire system as a medium to obtain the current online user's entire site information (login status information and other user-wide site information that needs to be processed).

Login process:

1. Log in to a site for the first time:

a) The user enters the username and password , Send a login request to the user verification center

b) Currently logged in to the site, through the webservice request, the user verification center verifies the legitimacy of the user name and password. If the verification is passed, a ticket is generated to identify the user of the current session, and the site identifier of the currently logged in sub-site is recorded in the user center, and finally

c) Return the obtained user data and ticket to the sub-site. If the verification fails, the corresponding error status code is returned.

d) According to the result returned by the webservice request in the previous step, the current sub-site logs in the user: if the status code indicates success, the current site saves the ticket through the cookie of this site, and the site records the user's Login status. If the status code indicates failure, the user will be given a corresponding login failure prompt.

2. In the logged-in state, the user goes to another page:

a) Verify the user's login status through the site's cookie or session: If the verification is passed, enter the normal site processing program; Otherwise, the user center verifies the user's login status (sends a ticket to the user verification center). If the verification is passed, local login processing is performed on the returned user information. Otherwise, it indicates that the user is not logged in.

Logout process:

a) The current logout site clears the user’s login status of the site and the locally saved user’s unique random ID

b) Through the webservice interface, clear the unique random id recorded in the entire site. The webservice interface will return, log out the javascript code of other logged-in sub-sites, and this site will output this code.

c) js code accesses the W3C standard logout script of the corresponding site

How to define class constants ：

Const

What is an abstract method:

A method without a method body is an abstract method. Use Keyword abstract to modify the data type of

ajax, Parameters:

1 .json 2.jsonp 3.xml 4.html

Three-level linkage:

Subclass calls parent class ID Recursion

Resolve primary key conflict Configuration file command

#auto_increment_offset =1

#auto_increment_increment=2

5.Why use session control

The http protocol is a stateless protocol that identifies users through session ID

6.What are the protocols

httpHTTPS ip

7 HTTPWhat is the protocol

Hypertext Transfer Protocol

8. What are XML and HTML;

XML is Extensible Markup Language

HTML is hypertext Markup language

9、What is jsonp

One way to make cross-domain requests uses

##10.yiiFramework widget

Small Widgets are basically used in views, and the yii\base\Widget::widget() method can be called in views to use widgets.

11.tpFramework configuration file

common/config.php

12.tpSingle letter method of the framework

C Get configuration value

D Instantiate Model class

L language definition function

I obtain input parameters

M is used to instantiate a Model without a model file

...

13 .Callback function

array_map, array_walk, array_reduce

The subclass calls the parent class statically Method

parent::$a

phpError level:

fatalerror fatal error: 0000 0000 0000 001 Open 1

warning warning error: 0000 0000 0000 010 open 2

NOTICE warning: 0000 0000 0001 000 open 8

while do while Difference:

while is to judge first and then execute

dowhile is to execute first and then judge;

Commonly used magic Method:

__desctruct(); __get(); __set(); __isset()

Construction method

Automatically called when instantiating the program __construct():

Destruction method, automatically called when the program ends __destrcut():

Magic constant:

function class file line method namespace

Subquery:

(Treat the inner query result as the comparison condition of the outer query)

#No need to order by to query the latest products

 selectgoods_id,goods_name from goods where goods_id = (select max(goods_id)fromgoods);

 

 

MySAM InnoDB Difference:

innodb has fast write operations, supports transactions, data row locks, and foreign keys, but does not support full-text indexes. InnoDB’s primary key range is larger, with a maximum of 2 times that of MyISAM.

MyISAM does not support transactions and foreign keys. It has fast read operations and supports data lock tables, GIS data, and full-text indexes (because it does not support Chinese word segmentation, the full-text index of MyISAM is actually useless).

The difference between session and cookie:

1. Session is on the server side, cookie On the client (browser)

2. The session is stored in a file on the server by default (not in memory)

3. The running of the session depends on the session id, and the session id is the existence of a cookie , that is to say, if the browser disables cookies, the session will also be invalid (but it can be achieved in other ways, such as passing session_id in the URL)

4. Session can be placed in files, databases, Or in memory.

5. Session is generally used for user verification.

What is the difference between interface and abstract class:

1, abstract classes and interfaces cannot be instantiated directly. If they are to be instantiated, abstract class variables must point to subclass objects that implement all abstract methods, and interface variables must point to class objects that implement all interface methods. .

2. Abstract classes must be inherited by subclasses, and interfaces must be implemented by classes.

3. Interfaces can only be used for method declarations. Abstract classes can be used for method declarations and method implementations.

  4, The variables defined in the interface can only be public static constants, and the variables in the abstract class are ordinary variables.

5. All abstract methods in an abstract class must be implemented by the subclass. If the subclass cannot implement all the abstract methods of the parent class, then the subclass can only Is an abstract class. Similarly, when a class implements an interface, if it cannot implement all interface methods, then the class can only be an abstract class.

6. Abstract methods can only be declared but not implemented. Interfaces are the result of design, and abstract classes are the result of reconstruction

  7, There can be no abstract method in an abstract class

    8, If there is an abstract method in a class, then this class It can only be an abstract class

9. The abstract method must be implemented, so it cannot be static or private.

10, Interfaces can inherit interfaces, and can inherit multiple interfaces, but classes can only inherit from a single root.

for Which is faster, loop or foreach loop?

When foreach loops the array, the pointer will automatically point to the next element. There is no need to calculate the array length. There is no conditional judgment and auto-incrementing variables, so the performance is definitely higher than for.

Delete all the data of males between 20-30 in the table:

Deleteform table name where sex =0 and age between 20 and 30;

What are the differences and benefits between static and pseudo-static?

Static web pages usually end with .html or htm, and the address corresponds to a file. This file actually exists in a directory on the website server.

Benefits: 1. When a visitor accesses a web page, the database is not read and the files corresponding to the website space are directly accessed. (Read files directly)

2. Purely static web pages are friendly to search engines and are the easiest to be included by search engines. (Easy to include)

3. Since the server does not need to do too much processing when accessing a web page, the pressure on the server is minimal, so it is easier to cope with high traffic volume. (Saving server pressure)

4. Some attacks against databases, such as SQL injection attacks, are often difficult to start from the address when facing static web pages. (High security)

Pseudo-static is actually dynamic, and the data processing process is the same as dynamic.

Benefits: In terms of the form of the URL, the pseudo-static address and the static one can be exactly the same. It is not easy for ordinary visitors to distinguish whether it is a real static address or a pseudo-static one (easy to include)

Alipay synchronous callback and asynchronous callback:

When a payment request is sent to the payment channel, the payment channel will quickly return a result. But this result only tells you that the call was successful, not that the deduction was successful. This is called synchronous call. Many novices will take this result as a successful payment, and they will be tricked. The result is that the payment success rate is extremely high, accompanied by a bunch of unexplained bad debt rates. Testers must pay special attention to the tampering of test data: amount, synchronization Return results, order number, etc.

There will be a callback address in the synchronous request parameters. This address is called by the payment channel after the deduction is successful. This is called an asynchronous call. General synchronization interface only checks whether the parameters are correct, whether the signature is correct, etc. The asynchronous interface will tell you the deduction result. Generally, asynchronous interfaces have a delay of less than 5 seconds. If the call is unsuccessful, it will be retried. Sometimes it is successful here, but the payment channel side does not receive a return, so it will continue to adjust. It is normal for the payment of one day to be called asynchronously until the next day. This is also something developers need to pay special attention to, not to treat it as a repeated payment. Testers should also test for repeated callbacks, which should only be valid once. This is not the biggest pitfall. Generally, on the payment channel side, you will only be notified if the payment is successful. If the payment fails, we won't tell you at all. On the other hand, why can’t I receive asynchronous results? Then you have to check it out. Synchronous results are unreliable, and asynchronous calls are unreliable. So how to determine the payment result? The final trick is to check the order and reverse check. Generally, the payment channel side will provide a reverse check interface. Regularly obtain the orders to be paid in the DB and call the reverse check interface on the payment channel side, and finally complete the orders with successful deductions on the payment channel side. Lose.

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