I'm having a problem with my customer's host. The host has configured the server in a way that php4 and php5 co-exist. If you want to use php4, you just have to use a .php extension for your files. If you want to use php5, you have to use a .php5 extension.
The problem with this configuration is that I'm using a system which has a codebase of 5000+ files all having a .php extension, but using native features of php5.
Is there a way to avoid having to rename every file to a .php5 extension and having to rewrite a small part of the codebase?
That depends on your host. You can override the handler for file types with this directive put in an .htaccess file, as long as your host hasn't disallowed it:
AddHandler php5-script .php