If you enable "Dedicated Application Pool" option on web site properties in DNP you need to do the following steps in order to make PHP scripts working on that web site:
1. Open "Local Security Settings" MMC snap-in (Start -> Administrative Tools -> Local Security Policy).
2. Navigate to "Security Settings -> Local Policies -> User Rights Assignments":
3. Add "IIS_WPG" group to the following policies:
4. Close MMC snap-in.
5. Restart IIS (Start -> Run... -> iisreset).