Variables in PHP

Overview

Variables in PHP are represented by a dollar sign followed by the variable name.

Variable names are case-sensitive.

A valid variable name starts with a letter or underscore, followed by any number of letters, numbers, or underscores.

$this is a special variable that cannot be assigned a value.

Variables are always assigned by value by default. If reference assignment is required, use &
Only named variables can be assigned by reference

<code>$foo = 25;
$bar = &$foo;      // 合法的赋值
$bar = &(24 * 7);  // 非法; 引用没有名字的表达式
</code>

Although there is no need to initialize variables in PHP, it is a good habit to initialize variables. Uninitialized variables have a default value for their type - Boolean variables have a default value of FALSE, integer and floating point variables have a default value of zero, string variables (such as those used in echo) have a default value of the empty string and arrays The default value of a variable is an empty array.

Predefined Variables

<code>超全局变量 — 超全局变量是在全部作用域中始终可用的内置变量
    $GLOBALS — 引用全局作用域中可用的全部变量
    $_SERVER — 服务器和执行环境信息
    $_GET — HTTP GET 变量
    $_POST — HTTP POST 变量
    $_FILES — HTTP 文件上传变量
    $_REQUEST — HTTP Request 变量
    $_SESSION — Session 变量
    $_ENV — 环境变量
    $_COOKIE — HTTP Cookies
$php_errormsg — 前一个错误信息
$HTTP_RAW_POST_DATA — 原生POST数据
$http_response_header — HTTP 响应头
$argc — 传递给脚本的参数数目
$argv — 传递给脚本的参数数组
</code>

Many predefined variables in PHP are "superglobal", which means they are available in the entire scope of a script. They can be accessed within a function or method without executing global $variable;.

Variable scope

Global variables are automatically invalid in functions

<code><?php
$a = 1; /* global scope */

function Test()
{
    echo $a; /* reference to local scope variable */
}

Test();
?>
</code>

Global variables in PHP must be declared as global

<code><?php
$a = 1;
$b = 2;

function Sum()
{
    global $a, $b;

    $b = $a + $b;
}

Sum();
echo $b;
?>
</code>

When used in functions, the second way to access variables in the global scope is to use Special PHP custom $GLOBALS array

<code><?php
$a = 1;
$b = 2;

function Sum()
{
    $GLOBALS['b'] = $GLOBALS['a'] + $GLOBALS['b'];
}

Sum();
echo $b;
?>
</code>

static variable

Another important feature of variable scope is static variables. Static variables only exist in the local function scope, but their values ​​are not lost when program execution leaves this scope.

<code><?php

function test()
{
    static $a = 0;
    echo $a;
    $a++;
}

for ($i = 0; $i < 10; $i++) {
    test();
} // 0123456789

?>
</code>

Variable variables

The variable name of a variable can be dynamically set and used

<code>$Bar = "a";
$Foo = "Bar";
$World = "Foo";
$Hello = "World";
$a = "Hello";

$a; //Returns Hello
$$a; //Returns World
	$$$a; //Returns Foo
$$$$a; //Returns Bar
$$$$$a; //Returns a
</code>