Photoshop learning experience PHP learning basics Page 1/2

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 then return the generated html code. Passed to the client, 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, while variables are case-sensitive. That is to say, $name, $NAME and $NaMe are three different variables.
PHP uses semicolons to separate simple statements.
PHP comments
PHP supports C, C++ and Shell script style comments, as follows:
// Single-line comments
/* */ Multi-line comments (Note: cannot be nested)
# Script comments
~~~~~~~ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ ~~~~~~~~~~~~~~~~~~~~
Constants
A constant is a fixed value, defined with a simple identifier. Constants are case-sensitive by default.
Constant identifiers are always uppercase by convention.
define() uses this function to define constants.
String (string) constants are divided into: built-in constants and custom constants.
Constants can only contain scalar data (boolean (Boolean), integer (integer), float (floating point number))
Built-in constants: constants provided by the PHP system that will not change in value on any page
PHP_OS: Displays the operating system version of the server
PHP_VERSION: Display the PHP version
Some common system constants
__FILE__: PHP file name, if it is a reference file, the reference file name is displayed
__LINE__: The number of lines of the PHP file
TRUE FALSE: A constant indicating true or false
E_ERROR: Specify Identify the most recent errors in the code
E_WARNING: Indicate the most recent warnings in the code
E_PARSE: Analyze where there are potential problems in the code
E_NOTICE: For unusual but not necessarily wrong places
Custom constants
Use define () to define constants
define("mycomputer", "IBM");
Define constants: mycomputer The value of the constant is IBM
defined("mycomputer");
Check whether the constant is defined, return 1 if it is defined, otherwise return empty
In variable
PHP, a dollar sign ($) 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 be output at the same time Two variable names
$4site = 'not yet'; //Illegal variable name; starts with a number Variables cannot start with a number
$_4site = 'not yet'; $i site is = 'mansikka'; // Legal variable name; You can use Chinese but it is not recommended to use
isset($var) //Check whether the variable is defined
unset($var) //Delete the variable $var
empty($ var) //Determine whether the value of a variable exists
echo $var //Empty
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Variable variable
A variable variable obtains the value of an ordinary variable as the variable name of the variable variable
< php
$a = 'hello'; // Ordinary variables
$$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 $hello"; //Output: hello world
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Constants are different from variables
There is no dollar sign ($) in front of constants;
Constants can only be used with the define() function Definition, not through assignment statements;
Constants can be defined and accessed anywhere regardless of the rules of variable scope;
Once defined, a constant cannot be redefined or undefined;
The value of a constant can only be a scalar
Data type
Four scalar types:
Boolean
Integer
Float (float, also called double)
String (string)
Two composite types:
Array
Object (object)
Finally, there are two special types:
Resource (resource)
NULL Empty
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:
$bool = TRUE; // Boolean type
$str = “foo”; // String
$int = 12; boolean (gettype gets the type of the variable)
echo gettype($str); // Output string
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Integer type
The integer value can be specified in decimal, hexadecimal or octal notation, and can be preceded by an optional symbol (- or +).
< php
$a = 1234; // Decimal number
$a = -123; // A negative number
$a = 0123; // Octal number (equal to decimal 83)
$a = 0x1a; // Ten Hexadecimal number (equal to decimal 26)
>
Floating point
Floating point number (also called float, double or real number) can be defined with any of the following syntax:
< php
$a = 1.234;
$a = 1.2e3;
$a = 7E-10;
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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 chapter will explain the string type 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 string and string "0"
None Array of member variables
Object without cells (only available in 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 later
Resource (Resource)
Resources are composed of specialized Function to create and use
Type cast
Type cast in PHP: Add the target type enclosed in parentheses before the variable to be converted.
The 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
$foo = 10; // $foo is an integer
$bar = (boolean) $foo; // $bar is a boolean
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