IIS impersonation

I am running Windows XP Pro, with SP2. I recently downloaded PHP
4.3.10, which installed successfully, to F:\PHP.
As suggested, I ensured that the IUSR_MachineName account had
appropriate access to the subfolders, EXE and DLL.
When I attempt to run a simple test PHP page, I get "Either a required
impersonation level was not provided, or the provided impersonation
level is invalid".
An ASPX test page runs successfully, with Anonymous Access turned
off..

How can I determine which impersonation level is involved?

Dave