Generování náhledu v PHP

<?php

function getOrCreateImg($original, $w, $h) {
    $small = $original . '-' . $w . '-' . $h . '.jpg';
    $tries = 0;

    # if the small image doesn't exist yet
    while (! file_exists($small)) {
        $lock = $small . '.lock';

        # if $lock dir exists but it is too old (more than 5 seconds; adjust this constant)
        if (is_dir($lock) && time() - filemtime($lock) > 5) {
            $del = $small . '.del.lock';
            # lock the $del to remove safely $lock
            if (mkdir($del)) {
                # check once more as we have lock
                if (is_dir($lock) && time() - filemtime($lock) > 5) {
                    # we will remove $lock
                    rmdir($lock);
                    # someone will create it again; it may as well be us
                }
                # unlock $del
                rmdir($del);
            } else {
                # somebody else is removing $lock, that is fine; we will continue
            }
        }

        # try to lock $lock
        if (mkdir($lock)) {
            # check once more that $small does not exist
            if (! file_exists($small)) {
                $tmp = $small . '.tmp';

                $img = open_img($original);
                # some long calculation here

                # save the image into $tmp file not to $small
                save_img_to($img, $tmp);

                # "create" $small "at once"
                rename($tmp, $small);
            }
            # unlock $lock
            rmdir($lock);
        } else {
            # somebody else is creating the small image; we will wait
            $start = time();
            while (!file_exists($small)) {
                usleep(100 * 1000); // 100ms
                # if we waited for more than 5 seconds
                if (time() - $start > 5) {
                    # try again; the next time it may be us who will generate the image
                    break;
                }
            }
        }

        # increment number of tries
        $tries++;
        # give up after 3 tries (15 seconds in total)
        if ($tries > 3) {
            header($_SERVER['SERVER_PROTOCOL'] . ' 500 Internal Server Error', true, 500);
            exit();
        }
    }

    # send the $small image which now exists
    readfile($small);
}