Assigning the return value of new by reference is deprecated
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To see why this idiom was used in PHP4, see this manual page (note that PHP4 is long dead and this link is to an archived version of the relevant page)
Perhaps the constructor of MDB2 has some code that uses a $variable =& new ClassName();
I recently moved a site using SimplePie (http://simplepie.org/) from a server that was using PHP 5.2.17 to one that is using PHP 5.3.2. It was after this move that I began receiving a list of error messages such as this one:
Deprecated: Assigning the return value of new by reference is deprecated in .../php/simplepie.inc on line 738
After reviewing several discussions of this issue, I cleared things up by replacing all the instances of =& new with = new in the simplepie.inc file.
I'm not experienced enough to know if this will work in all instances where these error messages are received but it worked in this particular case and it may be worth trying.