User-defined global variables

The scope outside of any function or class is the global scope. When a PHP script includes another (using include or require) the scope remains the same. If a script is included outside of any function or class, it’s global variables are included in the same global scope, but if a script is included from within a function, the variables in the included script are in the scope of the function.

Within the scope of a function or class method, the global keyword may be used to create an access user-defined global variables.

<?php

$amount_of_log_calls = 0;

function log_message($message) {
    // Accessing global variable from function scope
    // requires this explicit statement
    global $amount_of_log_calls;

    // This change to the global variable is permanent
    $amount_of_log_calls += 1;

    echo $message;
}

// When in the global scope, regular global variables can be used
// without explicitly stating 'global $variable;'
echo $amount_of_log_calls; // 0

log_message("First log message!");
echo $amount_of_log_calls; // 1

log_message("Second log message!");
echo $amount_of_log_calls; // 2

A second way to access variables from the global scope is to use the special PHP-defined $GLOBALS array.

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.

This means that the log_message() function could be rewritten as:

function log_message($message) {
    // Access the global $amount_of_log_calls variable via the
    // $GLOBALS array. No need for 'global $GLOBALS;', since it
    // is a superglobal variable.
    $GLOBALS['amount_of_log_calls'] += 1;

    echo $messsage;
}

One might ask, why use the $GLOBALS array when the global keyword can also be used to get a global variable’s value? The main reason is using the global keyword will bring the variable into scope. You then can’t reuse the same variable name in the local scope.