Due to new suexec changes, high security sites can not execute php files that have write permissions for other (e.g. o+w); this is a security measure to prevent someone else from injecting code into your php script and then getting it to execute as the site administrator. Check /var/log/httpd/suexec_log for any error messages regarding this problem. You should chmod your PHP scripts so other does not have write access by doing:
chmod o-w file.php
You can also do this recursively in the html folder of the site to all files by doing:
chmod -R o-w .