ドライバが読み込まれているかどうかの確認

<?php
$driver = 'SomeDriver';
$class  = 'My_Package_Driver_' . $driver;
$file   = str_replace('_', '/', $class) . '.php';

//check if it exists and can be loaded
if (!@fclose(@fopen($file, 'r', true))) {
    throw new My_Package_Driver_Exception(
        'Driver ' . $driver . ' cannot be loaded.'
    );
}

//continue with including the driver
require_once $file;

//...
?>