Argh, cPanel. What version of cPanel? I have a cPanel 56 install I'm hoping to deep-six before Christmas in favor of a real man's server without such fluff. 😛 😃
Do they have SuPHP installed with it? Seems likely. In that case, likely Apache is running as whoever you are in cPanel. Hopefully the files in question are also owned by you. Open the cPanel File Manager and look at the Permissions for myscript.php and let us know what those are.
Check the error logs (maybe just "Errors" in the 'Metrics' section).
The other possibility is a bad/missing directive in the Apache config or .htaccess.