The difference between global and $GLOBALS[""] in php_PHP tutorial

The difference between global and $GLOBALS[""] in php. I originally thought that global and $GLOBALS were the same except for the different writing methods. However, in actual use, I found that the difference between the two is still very big!

Look at the following example first:
PHP code

// Example 1
function test_global() {
global $var1, $var2;
$var2 =& $var1;
}
function test_globals() {
$GLOBALS['var3'] =& $GLOBALS['var1'];
}
$var1 = 5;
$var2 = $var3 = 0;
test_global();
Print $var2 ."n";
test_globals();
Print $var3 ."n";
?>

The fulfillment effect is:
0
5

How could this happen? Shouldn’t it be two 5s? How could there be 1 0 and 1 5?

Well, let’s save the above questions and analyze the principles of $GLOBALS and global in depth!

We all know that variables are actually the "codenames" of the corresponding physical memory in the code,

Quoting the explanation of $GLOBALS in the PHP manual:

Global variable: $GLOBALS
Note: $GLOBALS is available in PHP 3.0.0 and later.

An array consisting of all defined global variables. The variable name is the index into the array.

This is a "superglobal", or can be described as an active global variable.
In other words, $var1 and $GLOBALS['var1'] in the above code refer to the same variable, not 2 different variables!

Let’s analyze what global has done?

We all know that the variables generated by functions in PHP are private variables of the function, so the variables generated by global cannot escape this rule. Why do we say this? Look at the following code:
PHP code

function test() {
global $a;
unset($a);
}

$a = 1;
test();
print $a;
?>

Copy code
The fulfillment effect is:
1
Why does it output 1? Hasn’t $a been unset? Unset failed? PHP bug?

No, in fact unset works. It unsets $a in the test function. You can add it after the function
print $a;
In other words, global generates the alias variable "$a" for $a outside the test function, in order to differentiate it from the $a outside

Go back to Example 1 above and look at the code "$var2 =& $var1;" in test_global. The above is a reference assignment operation, that is, $var2 will point to the physical memory address pointed to by var1

So we come to the conclusion that the difference between global and $GLOBALS[] in the function is:

global generates an alias variable in the function that points to the external variable of the function, instead of the real external variable of the function. Once the address pointed by the alias variable is changed, some unexpected situations will occur, such as Example 1.

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

You can compare the following two examples:

global:

function myfunction(){
global $bar;
​ unset($bar);
}
$bar="someting";
myfunction();
echo $bar;
?>

Output: something

$global[]:

Function foo()
{
​ unset($GLOBALS['bar']);
}

$bar = "something";
foo();
echo $bar;
?>

Output: empty

PHP’s global variables are a little different from C language. In C language, global variables take active effect in functions unless they are covered by local variables. This can cause problems, as someone might carelessly mutate a global variable. Global variables in PHP must be declared global when used in functions.
Examples of applying global
$a = 1;
$b = 2;

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

$b = $a + $b;
}

Sum();
echo $b;
?>
The output of the above script will be "3". Global variables $a and $b are declared in the function, and all reference variables of any variable will point to the global variable

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