You need to setup your include path. PEAR packages rely on it being set correctly, so that their includes work properly:
set_include_path('.:/A_PATH/lib/php/');
You can also set that in your php.ini
, which is how it's done most times.
A howto to setup your include path can be found in the PEAR 1 manual: http://pear.php.net/manual/en/installation.checking.php