Pear Pager not found problem

I installed and rebuild apache with PEar, I installed Pager 2.4.8, I tried with this first test and work

require_once 'System.php';
var_dump(class_exists('System'));

but with Pager not found error ...

require_once '/Pager.php'; ( I also tried Pager/Pager.php path )
var_dump(class_exists('Pager'));