I'm not sure what a doctoral certificate is. Is that something different that you have in the UK? I am a doctoral student of psychology (Psy.D.) and I have to go to school for 5 to 7 years, complete two comprehensive exams, one assessment internship (I did my clinical internship during my Masters), and complete a dissertation. Once I successfully defend that, only then I can be called Doctor. A diploma, like Sunny said, is just a piece of paper-- the degree is the culmination of all the work that you do and the success of the dissertation.