Users and Permissions neccessary for PHP

[split63]

I have several web sites running on my server.
PHP works on all except for one site. I don't see any difference between the permission when compariing sites, but I have it stuck in my head that it must be a permission issue. I have a simple test php file and I have copied it to the root of each web site. It works on all the sites except for this one.

So my question is, which users and permission are actually required for a php file to be found and executed.
For example, there is the IUSR for anonymous access, what about:
II_WPS
SYSTEM
INERACTIVE
_WebApplication
_WebAnnoymous
etc.,

Thanks