At some point we should migrate away from mysql to either mysqli or even PDO. Here's one reference point: http://php.net/manual/en/mysqlinfo.api.choosing.php
Moving to mysqli is fairly painless. Recoding for PDO would be difficult especially given extensions.
There will be several benefits once we've changed in terms of better support for OO, transactions and stored procedures.
I'm not sure if this should be held to 5.0 instead of going into 4.7.