Fixing Drupal 7's "Call to undefined function filter_var()" on Gentoo

If you're getting PHP fatal this error while running your Drupal 7 on a Gentoo server a quick suggestion is check if your PHP has the filter extension enabled as it is required by Drupal 7 Core.

If you don't the only thing you need to do is follow those steps:

$ flagedit dev-lang/php +filter
$ emerge dev-lang/php

After emerge completes, just restart your http server and you're done!

For more info check:
http://drupal.org/node/947594
http://drupal.org/node/395744
http://drupal.org/node/593262