PHP fictional mutex

<?php
/**
  * While this is not a 100% proof solution for critical sections
  * it does demonstrate how a mutex could work.
  *
  */


$foo    = 0;
$locked = false; // This is our mutex (mutual exclusion) lock.

function a() {
global $foo;
global $locked;

    // Wait till the other thread finishes (blocking!):
    while($locked) { /* Do nothing (we call this busy-waiting) */ }

    $locked = true; // This will trigger the while loop for the other thread.
    $foo    = $foo + 2;
    $locked = false;
}

function b() {
global $foo;
global $locked;

    // Wait till the other thread finishes (blocking!):
    while($locked) { /* Do nothing (we call this busy-waiting) */ }

    $locked = true; // This will trigger the while loop for the other thread.
    $foo    = $foo + 2;
    $locked = false;
}

print $foo;
?>