The difference between super global variables $GLOBALS and global in PHP

The difference between super global variables $GLOBALS and global in PHP_PHP Tutorial

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Release： 2016-07-13 10:28:10

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This article shares the difference between the super global variables $GLOBALS and global in PHP.

1. Super global variable $GLOBALS

There are many superglobal variables in PHP. The following are all superglobal variables:

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

Official description:

$GLOBALS — refers to all variables available in the global scope.

A global combined array containing all variables. The name of the variable is the key of the array.

The global variables that have appeared can be obtained through the $GLOBALS array.

In the PHP life cycle, the so-called global variables defined outside the function body cannot be directly obtained inside the function.

$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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In the above example, to access external $foo you must use the $GLOBALS array. This also applies to external global variables that come in through include files.

global in php also has such a function. The difference between it and $GLOBALS is:

global generates an alias variable in the function that points to the external variable of the function, rather than the real external variable of the function.

$GLOBALS[] is indeed called an external variable, and it will always be consistent inside and outside the function.

For member variables in a class, functions in the class must use the $this-> method to access them, and cannot use the $GLOBALS method:

The role of global 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.

2. Example explanation

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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The execution result is:

0
5

Why not 2 5s but 1 0 and 1 5? Modify the example again:

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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The execution result only enters a 2;

1. $GLOBALS is an array automatically formed from all defined global variables. The variable name is the index into the array. That is, $GLOBALS['var1'] is the same variable as the variable $var1 outside the function, so after $GLOBALS['var1'] is deleted, the variable no longer exists and cannot be output.

Note: $GLOBALS is an automatic global variable. This means it works in all scripts. There is no need to use global $GLOBALS; within a function or method to access it.

2. "global $var1;" is an alias variable "$var1" that generates $var1 outside the function. It is not a real variable outside the function. It only exists inside the function, so even if the alias variable is used within the function Deletion will not affect external variables, but the values of variables external to the function can be modified.

Maybe some people always want to know the difference between this and that:

When learning PHP programs, including other programs, you can experiment by yourself and think based on the results. Sometimes it may be faster and more accurate than searching online. Let's talk about it next, what should we do if PHP accesses variables in the global scope?

Example 1: global defines global variables.

function test_global(){
    global $var1;
    $var1='ok';
    unset($var1);
}
test_global();
$var2=&$var1;
unset($var1);
echo $var2;

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Run the program yourself without giving the results. Variables inside the function can be accessed. It can be seen from the results that unset only disconnects the variable name from the variable value, and does not destroy the variable value immediately. Moreover, the global variables defined inside the function actually only use the alias inside the function externally, so we are outside $var1 can still be accessed.

Example 2: $GLOBALS accesses variables defined outside the function inside the function.

$waibu='out';
function ff(){
    echo $GLOBALS['waibu'];
}
ff();

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It will be an error to use $waibu directly inside the function.

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