PHP Learning 1 (Basic) Page 1/2_PHP Tutorial

WEB Application

When the client makes a request to the server program, the web server responds to the corresponding page according to the request. When the page contains a PHP script, the server will hand it over to the PHP interpreter for interpretation and execution, and will generate The html code is then sent back to the client, and the client's browser interprets the html code and finally forms a page in web format.
What PHP can do

PHP is mainly used in three areas:
PHP analyzer, a WEB server and a WEB browser.
PHP syntax structure

The lexical structure of a programming language refers to a collection of basic rules that govern how to write programs in the language.
User-defined function names or class names are not case-sensitive, and variables are case-sensitive. That is to say, $name, $NAME and $NaMe are three different variables.
PHP uses semicolons to separate simple statements.
PHP comments

PHP supports C, C++ and Shell script style comments, as follows:
// Single-line comments
/* */ Multi-line comments (Note: cannot be nested)
# Script Comments
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Constant

A constant is a fixed value , defined with a simple identifier, constants are case-sensitive by default.
Constant identifiers are always uppercase by convention.
define() uses this function to define constants.
String (string) constants are divided into: built-in constants and custom constants.
Constants can only contain scalar data (boolean (Boolean), integer (integer), float (floating point number))

Built-in constants: constants provided by the PHP system whose values ​​will not change on any page

PHP_OS: Display the operating system version of the server
PHP_VERSION: Display the PHP version

Some common system constants
__FILE__: PHP file name, if it is a reference file, the reference file name is displayed
__LINE__: The number of lines in the PHP file
TRUE FALSE: A constant indicating true or false
E_ERROR: Indicates the most recent error in the code
E_WARNING: Indicates the most recent warning in the code
E_PARSE: Analyze where there are potential problems in the code
E_NOTICE: For unusual but not necessarily wrong places

Custom constants
Use define() to define constants
define( "mycomputer", "IBM");
Define constant: mycomputer The value of the constant is IBM
defined("mycomputer");
Check whether the constant is defined, return 1 if defined, otherwise return empty

Variable
In PHP, a dollar sign ($) followed by a variable name represents a variable.Variable names are case-sensitive
< php
$var = 'Bob';
$Var = 'Joe';
echo "$var, $Var"; // Output "Bob, Joe" can output two variable names at the same time
$4site = 'not yet'; // Illegal variable name; starts with a number Variables cannot start with a number
$_4site = 'not yet'; // Legal variable name; starting with an underscore Can start with an underscore
$i site is = 'mansikka'; // Legal variable name; Can be in Chinese but not recommended
isset($var) // Check whether the variable is defined
unset($var) //Delete variable $var
empty($var) //Determine whether the value of a variable exists
echo $var //Empty
>

Variable variable
A variable variable obtains the value of an ordinary variable as the variable name of the variable variable
< php
$a = 'hello'; //Ordinary variable
$ $a = 'world'; //Variable variables Variable variables use the value of an ordinary variable as the name of the variable variable
echo "$a ${$a}"; //Output: hello world
echo "$a $hello"; //Output: hello world
>

Constants are different from variables
There is no dollar sign ($) in front of the constant;
Constant can only be used define( ) Function definition, not through assignment statements;
Constants can be defined and accessed anywhere regardless of the rules of variable scope;
Once a constant is defined, it cannot be redefined or undefined;
The value of a constant It can only be a scalar

Data type
Four scalar types:
Boolean
Integer (integer)
Float (floating point number, also (as double)
String
Two composite types:
Array (array)
Object (object)
Finally there are two special types:
Resource (resource)
NULL
PHP is a very weakly typed language.
In PHP, the type of a variable is usually not set by the programmer. Rather, it is determined at runtime (i.e., the value of the variable) based on the context in which the variable is used.
Example:
< php
$bool = TRUE; // Boolean
$str = “foo”; // String
$int = 12; //Integer
echo gettype($bool); // Output boolean (gettype gets the type of variable)
echo gettype($str); // Output string
>

Integer
Integer values ​​can be specified in decimal, hexadecimal or octal notation, preceded by an optional sign (- or +).
< php
$a = 1234; // Decimal number
$a = -123; // A negative number
$a = 0123; // Octal number (equal to decimal 83)
$a = 0x1a; // Hexadecimal number (equal to decimal 26)
>

Floating point type
Floating point number (also called floating point number, double precision number or real number ) can be defined using any of the following syntax:
< php
$a = 1.234;
$a = 1.2e3;
$a = 7E-10;
>

String
string is a series of characters. In PHP, characters are the same as bytes, which means there are a total of 256 different character possibilities. This also implies that PHP has no native support for Unicode. (The following dedicated chapter on string types will explain in detail)
< php $str = “hello world!”; >

Boolean
This is the simplest type. boolean represents a truth value, which can be TRUE or FALSE.
When other types are converted to boolean types, the following values ​​are considered FALSE:
Boolean value FALSE
Integer value 0 (zero)
Floating point value 0.0 (zero)
Blank Strings and strings "0"
Array with no member variables
Object without cells (only for PHP 4)
Special type NULL (including variables that have not been set)
All other values are considered TRUE (including any resources).

Array
Array is an important data type in PHP. A scalar can only store one data, while an array can store multiple data.
$my=array('1','2','abc','d');
Object (Object)
Object is an advanced data type that we will learn about later

Resource (Resource)
Resources are created and used by specialized functions

Type casting
Type casting in PHP: add parentheses before the variable to be converted target type.
Allowed casts are:
(int), (integer) - Convert to integer type
(bool), (boolean) - Convert to Boolean type
(float), (double), (real) - Convert to floating point type
(string) - Convert to string
(array) - Convert to array
(object) - Convert to object
< php
$ foo = 10; // $foo is an integer
$bar = (boolean) $foo; // $bar is a boolean
>

http://www.bkjia.com/PHPjc/319106.htmlwww.bkjia.comtruehttp: //www.bkjia.com/PHPjc/319106.htmlTechArticleWEB Application When the client makes a request to the server program, the web server responds to the corresponding page according to the request. When the page contains a PHP script, the server will hand it over to the PHP interpreter for interpretation...