ADODB use existing PDO connection

<?php
/**
 * The default database handle is a PDO object.
 * This will be used for normal database interaction.
 */
$db = new PDO('mysql:host=127.0.0.1;dbname=test', 'user', 'password');

/**
 * But if I run into legacy code which needs ADOdb to do something
 * I would like to use the existing PDO connection instead of wasting
 * resources making a second connection to the same database.
 */
require_once('adodb/adodb.inc.php');
require_once('adodb/drivers/adodb-pdo.inc.php');

class CustomAdodbPdo extends ADODB_pdo {
    function __construct($pdo) {
        try {
            $this->_connectionID = $pdo;
        } catch (Exception $e) {
            $this->_connectionID = false;
            $this->_errorno = -1;
            $this->_errormsg = 'Connection attempt failed: '.$e->getMessage();
            return false;
        }

        if ($this->_connectionID) {
            $this->dsnType = strtolower(PDO::ATTR_DRIVER_NAME);
            switch (ADODB_ASSOC_CASE) {
                case 0: $m = PDO::CASE_LOWER; break;
                case 1: $m = PDO::CASE_UPPER; break;
                default:
                case 2: $m = PDO::CASE_NATURAL; break;
            }
            $this->_connectionID->setAttribute(PDO::ATTR_CASE,$m);
            $class = 'ADODB_pdo_'.$this->dsnType;
            switch ($this->dsnType) {
                case 'oci':
                case 'mysql':
                case 'pgsql':
                case 'mssql':
                    include_once(ADODB_DIR.'/drivers/adodb-pdo_'.$this->dsnType.'.inc.php');
                    break;
            }
            if (class_exists($class))
                $this->_driver = new $class();
            else
                $this->_driver = new ADODB_pdo_base();

            $this->_driver->_connectionID = $this->_connectionID;
            $this->_UpdatePDO();
            return true;
        }
        $this->_driver = new ADODB_pdo_base();
        return false;
    }
}

$ADODB_NEWCONNECTION = 'CustomPdoFactory';
function &CustomPdoFactory() {
    global $db;

    $obj = new CustomAdodbPdo($db);
    return $obj;
}

  // The ADONewConnection() now uses the existing PDO connection "$db"
$adodb = &ADONewConnection();
?>