Table of Contents
PHP中超全局变量$GLOBALS和global的区别详解
一、超全局变量$GLOBALS
二、实例讲解
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PHP中超全局变量$GLOBALS和global的区别详解

Jun 20, 2016 pm 01:04 PM
php

 

PHP中超全局变量$GLOBALS和global的区别详解

一、超全局变量$GLOBALS

PHP超全局变量有很多,如下的都属于超全局变量(Superglobal):

$GLOBALS,$_SERVER,$_GET,$_POST,$_FILES,$_COOKIE,$_SESSION,$_REQUEST,$_ENV。

官方说明:

$GLOBALS — 引用全局作用域中可用的全部变量。

一个包含了全部变量的全局组合数组。变量的名字就是数组的键。

即出现过的全局变量,就可以通过$GLOBALS这个数组取得。

PHP生命周期中,定义在函数体外部的所谓全局变量,函数内部是不能直接获得的。

$foo = "Example content";
test();
function test() {
    $foo = "local variable";
    echo '$foo in current scope: ' . $foo . "<br>";
    echo '$foo in global scope: ' . $GLOBALS["foo"] . "<br>";
}
 
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如上的例子,要访问外部的$foo必须使用 $GLOBALS数组。对于通过include文件进来的外部全局变量也适用。

php中global也有这样的功能,它和 $GLOBALS 的区别在于:

global在函数产生一个指向函数外部变量的别名变量,而不是真正的函数外部变量。

$GLOBALS[]确确实实调用是外部的变量,函数内外会始终保持一致。

对于类中的成员变量,类中函数必须使用$this->的方式访问,不能用$GLOBALS方式:

global的作用是定义全局变量,但是这个全局变量不是应用于整个网站,而是应用于当前页面,包括include或require的所有文件。

二、实例讲解

function t1() {
    global $var1, $var2;
    $var2 = &$var1;
}
function t2() {
    $GLOBALS['var3'] = &$GLOBALS['var1'];
}
$var1 = 5;
$var2 = $var3 = 0;
t1();
print $var2 ."\n";
t2();
print $var3 ."\n";
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执行结果为:

0
5
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为什么不是 2个5 而是 1个0和1个5 呢?

再修改一下例子:

function t1() {
    global $var1;
    $var1 = 2;
    unset($var1);
}
function t2() {
    $GLOBALS['var1'] = 3;
    unset($GLOBALS['var1']);
}
$var1 = 1;
t1();
print $var1 . "\n";
t2();
print $var1 . "\n";
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执行结果只输入一个2;

$GLOBALS是由所有已定义全局变量自动形成的数组。变量名就是该数组的索引。

即$GLOBALS['var1']与函数外部的变量$var1是同一个变量,所以将$GLOBALS['var1'] 删除后,该变量已不存在,所有无法输出了。

注:$GLOBALS是自动全局变量。这意味着它在所有的脚本中都有效。在函数或方法中不需要使用 global $GLOBALS; 来访问它。

"global $var1;"是产生了函数外部$var1的别名变量"$var1"

它不是真正的函数外部变量,他只存在于函数的内部,所以即使在函数内将别名变量删除也不会影响外面的变量,但是可以修改函数外部变量的值。

或许有的人总想知道这个或那个的区别:在php程序,包括其他程序的学习中,自己动手实验,根据结果加上的思考,有的时候比上网查找可能会来得更快一些,更准确一些。下面我们来讲一下,php在全局范围内访问变量要怎么办?

例一:global定义全局变量。

function test_global() {
    global $var1;
    $var1 = 'ok';
    unset($var1);
}
test_global();
$var2 = &$var1;
unset($var1);
echo $var2;
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先不给出结果,自己运行一下程序。函数内部的变量可以访问到了。

从结果可以看出,unset只是断开变量名与变量值连接,并没有马上销毁变量的值,而且在函数内部定义的全局变量,实际在外部只是使用了函数内部的别名而已,所以我们在外面依然可以访问$var1。

例二:$GLOBALS在函数内部访问函数外面定义的变量。

$waibu = 'out';
function ff() {
    echo $GLOBALS['waibu'];
}
ff();
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直接在函数内部使用$waibu是会出错的。


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