While surfing for the exact same problem with squirrelmail and Fedora 12 I found this post:
"Nevermind, I found the answer:
fedora has multiple .ini files that load its php extensions.
Adding the 'extension=php_mapscript.so' line to the gd.ini file in
'/etc/php.d' directory solved the problem."
This fixed my issue as well.
Hope that helps!!