Running a php script via ajax, but only if it is not already running

Question

My intention is this.

My client.html calls a php script check.php via ajax. I want check.php to check if another script task.php is already being run. If it is, I do not

Answer 1

You can get an exclusive lock on the script itself for the duration of the script running

Any other attempts to run it will end as soon as the lock() function is invoked.

//try to set a global exclusive lock on the file invoking this function and die if not successful
function lock(){
  $file = isset($_SERVER['SCRIPT_FILENAME'])?
    realpath($_SERVER['SCRIPT_FILENAME']):
    (isset($_SERVER['PHP_SELF'])?realpath($_SERVER['PHP_SELF']):false);
  if($file && file_exists($file)){
    //global handle stays alive for the duration if this script running
    global $$file;
    if(!isset($$file)){$$file = fopen($file,'r');}
    if(!flock($$file, LOCK_EX|LOCK_NB)){
        echo 'This script is already running.'."\n";
        die;
    }
  }
}

Test

Run this in one shell and try to run it in another while it is waiting for input.

lock();

//this will pause execution until an you press enter
echo '...continue? [enter]';
$handle = fopen("php://stdin","r");
$line = fgets($handle);
fclose($handle);