The top answer is already good. I'm providing an alternative that I use. I'm simply declaring the time period instead of the achieved academic degree.
PhD student of Mathematics, Department of Mathematics, Foo Baz University, BB, August 2011 – March 2015
Edit: To make it clearer: you can put "work experience", "studies" and "academic degrees" in different sections of your CV. That way it will be clear, that you have been a PhD student for a long time, but your highest archivement it your Bsc/Msc degree.