Network services almost never expose actual paths on the server's hard disk drive and even if they could it isn't a behaviour you can rely on. So the fact that your file is located at /admin/stock_update.php
in the FTP server doesn't say much about actual location on disk, which is what local command-line utilities expect.
In PHP, you can find path on disk of current file with the __FILE__
magic constant. You can create a test script:
<?php
var_dump(__FILE__);
... upload it to the same FTP location and execute through the web server. If that's not an option because files in your FTP account in not visible from the web you can run the file from cron and check the email.