Ensure a PHP script can only have 1 (or N) instances running at a time

script_run_once.php

<?php

//Use PHP semaphores to ensure only one (or $max) instances of a script can run at once.
//Semaphores can auto-release if the script crashes, unlike a lock file or database flag which require the script to be functional to release them

//USAGE: Simply include this file at the top of your script. When run, it will die if it finds another instance of itself already running.
// include_once('/path/to/script_run_once.php');


//All we need is a unique 4-digit integer to represent our script. I will hash the current script file path to get this, but you can hard-code $key if you like
//Note $_SERVER['SCRIPT_FILEPATH'] will be the name of the file which has include()-ed script_run_once.php
//Also note with only 9999 possible keys, hash collisions between different strings are unlikely but not impossible

$bignum = hexdec( substr(sha1($_SERVER['SCRIPT_FILENAME']), 0, 15) ); 
$key = $bignum % 9999;

$max = 1; //max instances
$permissions = 0666;
$autoRelease = 1;

//make or get the semaphore matching $key
$semaphore = sem_get($key, $max, $permissions, $autoRelease);

//Try to acquire the semaphore. This will fail if the semaphore is already acquired by >= $max instances of this script
$sem = sem_acquire($semaphore,true);
if($sem === false) { //already running
    die('This script is already running');
}