The global keyword

The global keyword is used to bind a variable from a global scope into a local scope. The keyword can be used with a list of variables or a single variable. A local variable will be created referencing the global variable of the same name. If the global variable does not exist, the variable will be created in global scope and assigned null.

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

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

    $b = $a + $b;
} 

Sum();
echo $b;
?>

3

By declaring $a and $b global within the function, all references to either variable will refer to the global version. There is no limit to the number of global variables that can be manipulated by a function.

A second way to access variables from the global scope is to use the special PHP-defined $GLOBALS array. The previous example can be rewritten as:

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

function Sum()
{
    $GLOBALS['b'] = $GLOBALS['a'] + $GLOBALS['b'];
} 

Sum();
echo $b;
?>

The $GLOBALS array is an associative array with the name of the global variable being the key and the contents of that variable being the value of the array element. Notice how $GLOBALS exists in any scope, this is because $GLOBALS is a superglobal. Here's an example demonstrating the power of superglobals:

<?php
function test_superglobal()
{
    echo $_POST['name'];
}
?>

Note: Using global keyword outside a function is not an error. It can be used if the file is included from inside a function.

Using static variables

Another important feature of variable scoping is the static variable. A static variable exists only in a local function scope, but it does not lose its value when program execution leaves this scope. Consider the following example:

<?php
function test()
{
    $a = 0;
    echo $a;
    $a++;
}
?>

This function is quite useless since every time it is called it sets $a to 0 and prints 0. The $a++ which increments the variable serves no purpose since as soon as the function exits the $a variable disappears. To make a useful counting function which will not lose track of the current count, the $a variable is declared static:

<?php
function test()
{
    static $a = 0;
    echo $a;
    $a++;
}
?>

Now, $a is initialized only in first call of function and every time the test() function is called it will print the value of $a and increment it.

Static variables also provide one way to deal with recursive functions. The following simple function recursively counts to 10, using the static variable $count to know when to stop:

<?php
function test()
{
    static $count = 0;

    $count++;
    echo $count;
    if ($count < 10) {
        test();
    }
    $count--;
}
?>

Prior to PHP 8.3.0, static variables could only be initialized using a constant expression. As of PHP 8.3.0, dynamic expressions (e.g. function calls) are also allowed:

<?php
function foo(){
    static $int = 0;          // correct 
    static $int = 1+2;        // correct
    static $int = sqrt(121);  // correct as of PHP 8.3.0

    $int++;
    echo $int;
}
?>

As of PHP 8.1.0, when a method using static variables is inherited (but not overridden), the inherited method will now share static variables with the parent method. This means that static variables in methods now behave the same way as static properties.

<?php
class Foo {
    public static function counter() {
        static $counter = 0;
        $counter++;
        return $counter;
    }
}
class Bar extends Foo {}
var_dump(Foo::counter()); // int(1)
var_dump(Foo::counter()); // int(2)
var_dump(Bar::counter()); // int(3), prior to PHP 8.1.0 int(1)
var_dump(Bar::counter()); // int(4), prior to PHP 8.1.0 int(2)
?>

References with global and static variables

PHP implements the static and global modifier for variables in terms of references. For example, a true global variable imported inside a function scope with the global statement actually creates a reference to the global variable. This can lead to unexpected behaviour which the following example addresses:

<?php
function test_global_ref() {
    global $obj;
    $new = new stdClass;
    $obj = &$new;
}

function test_global_noref() {
    global $obj;
    $new = new stdClass;
    $obj = $new;
}

test_global_ref();
var_dump($obj);
test_global_noref();
var_dump($obj);
?>

The above example will output:

NULL
object(stdClass)#1 (0) {
}

A similar behaviour applies to the static statement. References are not stored statically:

<?php
function &get_instance_ref() {
    static $obj;

    echo 'Static object: ';
    var_dump($obj);
    if (!isset($obj)) {
        $new = new stdClass;
        // Assign a reference to the static variable
        $obj = &$new;
    }
    if (!isset($obj->property)) {
        $obj->property = 1;
    } else {
        $obj->property++;
    }
    return $obj;
}

function &get_instance_noref() {
    static $obj;

    echo 'Static object: ';
    var_dump($obj);
    if (!isset($obj)) {
        $new = new stdClass;
        // Assign the object to the static variable
        $obj = $new;
    }
    if (!isset($obj->property)) {
        $obj->property = 1;
    } else {
        $obj->property++;
    }
    return $obj;
}

$obj1 = get_instance_ref();
$still_obj1 = get_instance_ref();
echo "\n";
$obj2 = get_instance_noref();
$still_obj2 = get_instance_noref();
?>

The above example will output:

Static object: NULL
Static object: NULL

Static object: NULL
Static object: object(stdClass)#3 (1) {
  ["property"]=>
  int(1)
}

This example demonstrates that when assigning a reference to a static variable, it is not remembered when the &get_instance_ref() function is called a second time.