All variables names need to be selfdescriptive. I know this $d is already in this class everywhere, but for me we should rename it $driver or something that descrives clearly the what this variable contains. A variable name of less than 3 chars is usually not good name. I fact is one of the things that you can scan for with the PHP Mess detector (http://phpmd.org/rules/naming.html#shortvariable). You don't need to change it now, but lets consider it for the future pulls.