You may also modify the AddType directives in httpd.conf rather than edit mime.types, which will accomplish the same thing:
AddType application/x-httpd-php .php .phtml .php3
To use .php3 files with PHP4 in (at least) Apache edit your mime.types and comment out the php3 mime types and add php3 to the line reading:
application/x-httpd-php phtml pht php
So it will read:
application/x-httpd-php phtml pht php php3
Restart Apache to be sure it picks up the new settings.