It seems PHP is installed as a CGI and not as module.
Check whether the file permission is set to 700. If your file permission is set to something else (e.g., 644), you will get a 500 - Internal Server Error. Note: Setting the file permission to 711 or 755 will not give you a 500 error
When the file/directory permissions are set correctly, and you are still getting a 500 error, open the file using pico and save it (Ctrl+o). This simple step may solve your problem because it converts Windows CR/LF (carriage return/line feed) to Unix LF (line feed) or Mac CR to Unix LF.
Make sure you upload the php files in ascii mode and not in binary mode.