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For iis, you 'remap the executable for html to the executable for asp'
What that means in IIS, go to the part where it has the different paths for different file extensions. Select the one for asp, copy that, and then go to the one for html. Past the one for asp there, save the configuration, and you're done.
Though, I might be missing the question...I don't think you can parse both asp and php the same way on any platform, eg, have the server check if an html page is asp or php before serving.
What you could do if that's what you really need is process all .htm's for asp, and then process all .html's for php. :)