Today a VPS I own ran in to a slight issue, every PHP we tried to run failed with a 500 Internal Server Error.
After a few hours of diagnostics and upgrades I finally figured out what the issue was, the SuPHP module was attempting to run php5 on the server, unfortunately php5 was a symlink to the php-cli executable and not the php-cgi executable as it was expecting.
The fix was simple in the end, issue the following command.
ln –s /usr/local/php /usr/local/php5
Hope this saves someone else some head scratching!