Try entering this into your httpd.conf file:
php_value include_path "/Library/WebServer/Documents/"
You will find a number of these <IfModule> ... </IfModule> entries in your config file. Place it after one of the closing </IfModule> statements. Change the path to whatever you want the include path to be. Also, if you have any virtual hosts set up you can add this inside the <VirtualHost *> ... </VirtualHost> statements. Again, change the path to correspond to the Virtual Host's path.
Hope this helps.