I've just got an error.
When I try to assign an object like this:
$obj_md = new MDB2();
In PHP5 this idiom is deprecated
$obj_md =& new MDB2();
You sure you've not missed an ampersand in your sample code? That would generate the warning you state, but it is not required and can be removed.
To see why this idiom was used in PHP4, see this manual page.