For PHP, in IIS for that website, in the application mappings (Home Diry tab, click Configuration in the lower part of the dialog), ensure that for the .php extension the option "Check that file exists" is set -- that should ensure that your custom error page is displayed for a not-found .php file.
As for the .htm/.html page, I was wrong giving you "htttp://domain.com/page.asp" as the format for a URL: it has to be of the form "/domain.com/page.asp" -- in other words the location of the error file relative to the "root" of the virtual directory for that domain (I was doing it from memory not looking -- slapped wrist!).
Chris at TDMWeb.com
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Given that your 'asp error' pages work with a .asp extension, but not with the other file extensions, I'd try mapping the asp.dll to the other file extensions (.htm and .html at least, mapping .php to asp.dll would not be a good idea) and see what happens. It may well work.
(ISM: Website Properties, Home Directory, Configuration, App Mappings).
The last 2 suggestions look promising - I did a few searches and came up with this: http://www.windowswebsolutions.com/A...rticleID=22278
Which also points at the "Check that file exists" option - I have passed your advice to my host, and will let you know the outcome!
I said I would let you know the outcome... well, the problem is almost certainly due to the "check that files exists" problem, however it may be a month or so more before my web host gets around to implementing the solution... I'm still waiting for a response form my last 2 updates to this trouble ticket.
However I also found that moving my error handler script "handler.asp" to the root of my website (it was in a '/404/' subfolder) gets around this problem for all but the .PHP extension.