A 500 level error is an error from the server. Does your hosting support php? If so you should create a phpinfo page by making a new file called phpinfo.php and ensuring that it has this line of code in it:
<?php phpinfo(); ?>
After you create this file upload it to your server and browse to it. It should let you know about the details of your php server setup. This must be correctly setup and working before you will be able to use any php pages on your site. Please post back and let us know what you discover.