MySQLi: Cast port to integer, just to be safe

Follow up on my PR from yesterday about specifying a custom port with MySQLi: it seems that port as string [may create problems](http://www.php.net/manual/en/mysqli.construct.php#103521) in PHP 5.3.3. That PR is making sure the port is an integer.

Sorry for not having introduced this in the previous PR... Also, sorry for showing my git noobism (3 commits for that single change, oh my...)