just a thought, no solution
Hi,
the basic rule for http servers and clients: the server describes the data to the client by sending a proper mime type, and the client obeys.
One browser has chosen to do things differently, so both the file name and the first few bytes of data are used to determine the data type, rather than using the supplied one (The same company is notorious for not providing up-to-date mime maps with their webservers ... because the client knows how to do it)
Maybe this particular client has got a strange rule in their system that associates a php file with something else.
That person could try with a different, more standards-conforming browser and see whether the problem persists...
Musicman