make PHP wait until a function completes?

Is there any way to make PHP wait until a function returns before continuing?

This is my code:

<?php

    set_time_limit(0);

    function waitforchange($nof) {
        $lfilemod=filemtime($nof);
        while(filemtime($nof) == $lfilemod) {
            clearstatcache();
            usleep(10000);
        }
    }

    waitforchange('./blahblah.txt')
    sleep(5);
    echo 'done';

?>

It is supposed to wait until blahblah.txt changes, wait another five seconds after that, then print out "done", however, it prints out "done" after five seconds, regardless whether the file actually changed.