// Created on savesnippets.com · https://savesnippets.com/ocwFUe4l8m2cdy
<?php
function increment(string $file, int $by = 1): int {
    $fh = fopen($file, 'c+');
    flock($fh, LOCK_EX);
    $current = (int)stream_get_contents($fh);
    $next    = $current + $by;
    rewind($fh);
    ftruncate($fh, 0);
    fwrite($fh, (string)$next);
    fflush($fh);
    flock($fh, LOCK_UN);
    fclose($fh);
    return $next;
}

$jobId = increment('/var/lib/myapp/jobs.counter');
echo "Assigned job #$jobId\n";