My PHP files generate a "500 server error" message.

The most common reason for "500 server error" message, output by a PHP script, are incorrect file permissions. You need to make sure that the PHP script itself and every other file or directory on the account does not have world-writable permissions (666, 777).

Our servers are running under SuExec environment and scripts are run with the username of their owner, and not with the user of the web server ("apache"). Thus, if a script has to write to a file/directory, world-writable permissions are not needed.
A file/directory with world-writable permissions is considered insecure, and cannot be used/executed.