Here is a straight way to get a fully functionnal GeoIP module for PHP5 on Debian:
apt-get install php5-geoip geoip-database-contrib
The second package is actually a download facility to fetch the various free databases from Maxmind. Once it’s done, edit the php.ini configuration file to append:
[geoip]
geoip.custom_directory = "/usr/share/GeoIP/"
Reload the web server and it should now work.