yeah also agree with lobobr.
i don't think there should be a problem running both php and asp on the same site.
Now if your are trying to run asp and php on the same file, then you'd have to change the configuration on your server in how to interpretate (?) the file extension. I've never tried this, but i would assume you could make .html, .php, *.asp files parse both ASP and PHP. No guarantees though.
Otherwise, just make sure you are pointing the forms/links to the right page with the correct extension.