This post specifies the default php includes path on a linux distribution. The one I used was gentoo.
This is the path that will be globally included in all your php projects.
It is:
include_path = ".:/usr/share/php5:/usr/share/php"
If you want to change this path, just change the above line in php.ini and restart apache or other web server.
php.ini can be found at:
/etc/php/apache2-php5.5/php.ini