The use and difference between global and $GLOBALS[] in PHP-PHP Tutorial-php.cn

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The use and difference between global and $GLOBALS[] in PHP

巴扎黑

Nov 11, 2016 pm 03:00 PM

global php

The scope of a variable is the context in which it is defined (that is, its effective scope). Most PHP variables have only a single scope. This single scope span also includes files introduced by include and require. For example:

Php code

<?php  
$a = 1;  
include &#39;b.inc&#39;;  
?>

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Here the variable $a will take effect in the include file b.inc. However, in user-defined functions, a local function scope will be introduced. Any variables used inside a function will be restricted to the local function scope by default. For example:

Php code

<?php  
$a = 1; /* global scope */ 
   
function Test()  
{  
    echo $a; /* reference to local scope variable */ 
}  
   
Test();  
?>

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This script will not have any output because the echo statement refers to a local version of the variable $a, and it is not assigned a value in this scope. You may notice that PHP's global variables are a little different from C language. In C language, global variables automatically take effect in functions unless overridden by local variables. This may cause some problems, someone may accidentally change a global variable. Global variables in PHP must be declared global when used in functions.

global keyword

First, an example of using global:

Example #1 Using global

Php code

<?php  
$a = 1;  
$b = 2;  
   
function Sum()  
{  
    global $a, $b;  
   
    $b = $a + $b;  
}  
   
Sum();  
echo $b;  
?>

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The output of the above script will be "3". After global variables $a and $b are declared in a function, all references to either variable point to its global version. PHP has no limit on the maximum number of global variables that a function can declare.

The second way to access variables in the global scope is to use special PHP custom $GLOBALS array. The previous example can be written as:

Example #2 Use $GLOBALS to replace global

Php code

<?php  
$a = 1;  
$b = 2;  
   
function Sum()  
{  
    $GLOBALS[&#39;b&#39;] = $GLOBALS[&#39;a&#39;] + $GLOBALS[&#39;b&#39;];  
}  
   
Sum();  
echo $b;//3  
?>

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$GLOBALS is an associative array, each variable is an element, the key name corresponds to the variable name, and the value corresponds to the variable Content. $GLOBALS exists in the global scope because $GLOBALS is a superglobal variable. The following example shows the use of super global variables:

Example #3 Example demonstrating super global variables and scope

Php code

<?php  
function test_global()  
{  
    // 大多数的预定义变量并不 "super"，它们需要用 &#39;global&#39; 关键字来使它们在函数的本地区域中有效。  
    global $HTTP_POST_VARS;  
   
    echo $HTTP_POST_VARS[&#39;name&#39;];  
   
    // Superglobals 在任何范围内都有效，它们并不需要 &#39;global&#39; 声明。Superglobals 是在 PHP 4.1.0 引入的。  
    echo $_POST[&#39;name&#39;];  
}  
?>

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global and $GLOBALS[] difference

$GLOBALS['var' ] is the external global variable itself, global $var is the reference or pointer of the same name of the external $var

Php code

<?php   
$var1 = 1;   
function test(){   
unset($GLOBALS[&#39;var1&#39;]);   
}   
test();   
echo $var1;   
?>

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Result: Notice: Undefined variable: var1. Because the variable var1 was deleted by the unset function.

Php code

<?php   
$var1 = 1;   
function test(){   
global $var1;   
unset($var1);   
}   
test();   
echo $var1;   
?>

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Result: Print 1

Only the alias reference is deleted, and its value has not been changed in any way.

global $var is actually &$GLOBALS['var'], which is just an alias for calling external variables.

$var1 and $GLOBALS['var1'] in the above code refer to the same variable itself.

The function of PHP's global variable is to define global variables, but this global variable does not apply to the entire website, but to the current page, including all files in include or require.

Conclusion:

1.$GLOBALS['var'] is the external global variable itself

2.global $var is the reference or pointer of the same name of the external $var.

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