global keyword and $GLOBALS usage in php

First let’s look at a piece of code

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

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

Test ();
?>

This script will not have any output, because the echo statement references a local version of the variable $a, and within this scope, it has not been assigned a value. 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, the code is as follows:

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

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

     $b  =  $a  +  $b ;
}

Sum ();
echo  $b ;
?>

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 will 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 a special PHP custom $GLOBALS array. The previous example can be written as:

Example #2 Use $GLOBALS instead of global, the code is as follows:

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

function  Sum ()
{
     $GLOBALS [ &#39;b&#39; ] =  $GLOBALS [ &#39;a&#39; ] +  $GLOBALS [ &#39;b&#39; ];
}

Sum ();
echo  $b ;
?>

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

Example #3 Example demonstrating superglobal variables and scope

<?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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