// Created on savesnippets.com · https://savesnippets.com/1rqKavYv7FaG8M
<?php
function runIfNotLocked(string $lockFile, callable $job): bool {
    $fh = fopen($lockFile, 'c');
    if (!$fh) throw new RuntimeException("Cannot open $lockFile");
    if (!flock($fh, LOCK_EX | LOCK_NB)) {
        fclose($fh);
        return false;       // another instance is already running
    }
    try {
        $job();
        return true;
    } finally {
        flock($fh, LOCK_UN);
        fclose($fh);
    }
}

$ran = runIfNotLocked('/tmp/myjob.lock', function () {
    // Expensive cron task — runs only when no other copy holds the lock.
});
if (!$ran) error_log('myjob: skipped — another instance is running');