PHP won't redirect to error page

Our site's setup: We're hosted on an NT server running IIS. Our site is set to parse .shtml files as PHP since it made for an easier upgrade. Everything works fine except the problem below. We asked our host's support folks to redirect 404 errors to an error page to make the experience a little easier for visitors. Here's an interesting quirk. When you type in a nonexistent location like: http://www.digitalhit.com/bubba.html (notice that's html not shtml) or http://www.digitalhit.com/bubba the site redirects to the new error page just fine. If the file that doesn't exist is an .shtml file or .php3 file it tosses out a "PHP error: Fatal error: Unable to open F:\users\site\www\htdocs\bubba.shtml in - on line 0 No input file specified." instead of the error page. Any idea why PHP would jump over IIS's error redirect? Any solutions? -- Ian Evans