1. If you have an .htaccess file and it uses mod_rewrite (Joomla, WordPress, usually do), make sure it has the line
RewriteBase /
above any rewrites
2. All .php files must be permission less than 0777 in the last two digits (ie no write permission for group/world). 0755 should work. You can go to cPanel --> Error Log to see if it says something like 'index.php is group writeable'