Talk about global variables in PHP

As we all know, in PHP functions, if you want to use global variables, one is to use super global variables $GLOBALS, and the other is to use in the function Global keyword declaration, everyone knows how to use the super global variable $GLOBALS. Today let’s talk about using the global method to declare variables.

Let’s look at the following example:

$a = 1;
$b = 2;
function test() {
    global $a, $b;
}
test();

When we execute the test() function, what is the status of the variables in the memory? As shown in the figure below

#We declared the test function. When the test() function is called, global will declare two local variables $a, $b in the function, and Point to global variables $a and $b respectively. At this time, temporary variables $a and $b are equivalent to references to global variables $a and $b. So when we modify the values ​​of local variables $a and $b in the function, it is equivalent to modifying the value of the global variable.

$a = 1;
$b = 2;
function test() {
    global $a, $b;
    $a = 11;
    $b = 12;
    unset($a);
    unset($b);
}
test();
echo $a;
echo &#39;<br>&#39;;
echo $b;

Print output on the browser:

11
22

The above code also uses unset(), and the values ​​​​of global variables $a, $b can be output through echo output, which also shows that test The $a and $b declared through the global keyword in the () function are only local variables, and unset() only disconnects the references between local variables and global variables.

Let's look at another situation. What if the variables we declare in the test function are not in the global variables?

Let’s talk about the conclusion first: If a variable declared using the global keyword in a function does not have a variable with the same name in the global scope, then a global variable with the same name will be created first, and then a local variable will be created in the function. Point the reference of the global variable to the local variable.

function test() {
    global $a, $b;
    $a = 11;
    $b = 22;
    unset($a);
    unset($b);
}
echo $a;
echo &#39;<br>&#39;;
echo $b;

Output through the browser:

11
22

In the above code, we use global to declare the variables $a, $b in the test function, although the global scope does not declare $ a and $b, but global will still create global variables $a and $b first, then create $a, $b in the test function, and then point the reference of the global variable to the local variable of the test function. So even if we unset $a and $b in the function, we can still print out $a and $b after the function is executed, because there are already two variables in the global variable.

So we can finally draw the following conclusion:

When using global to declare a variable in a function, it will first check whether there is a variable with the same name in the global scope, and if so, it will The reference of this global variable points to a local variable of the same name declared using global. If there is no variable with the same name in the global scope, create a global variable with the same name first, then create a local variable, and then point the reference of the global variable to the local variable.

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