I'm running Linux Suse 8.1 with apache as my webserver. If you are running some version of Linux, you would probably find the mime file in a similar location. At any rate, under /etc/httpd/mime.types, there should be a entry similar to "applicatioin/x-httpd-php phtml pht php". That generally takes care of the local clients, however, in the same directory, in httpd.conf there are several entries regarding php. Look for something like:
<IfModule mod_php3.c>
AddType application/x-httpd-php3 .php3
<IfModule>
This is for the previous version of php, if you are running version 4.x then look for (usually, they are close together).
<IfModule mod_php4.c>
AddType application/x-httpd-php .php
</IfModule>
If you are running Windows, you may have to look in your systems directory for anything that says something about "Mime"?
Hope this helps?