PHP __DIR__ working locally but not on production

Question

I have a script in a Wordpress plugin and I am trying to incorporate logging using fwrite().

My code is like this:

 $log_url =  __DIR__ . '/logs/update-email-publish-tasks/' . date("n.j.Y") . '.txt';
    $logfile = fopen($log_url, "w"); 
    fwrite($logfile, 'Starting tasks at ' . date("H.i.s") . "\n");

//A bunch of code that logs stuff;

    fwrite($logfile, "\n" . 'Finished at !' . date("H.i.s") . "\n");
    fclose($logfile);

This is working fine locally but does not work on production. I am wondering if I need to use something besides __DIR__? I also tried WP_CONTENT_DIR which did not work.

FWIW I am hosting production on Pantheon.