A 620 is low. I would cancel your score and take again, but ONLY if you do something this time in terms of studying that's different -- like getting a different book, or taking a different course online. What you have been doing didn't work, so DO SOMETHING DIFFERENT, cancel your score, and take again.
Also, sometimes the GRE is an easier test, especially if you're stronger in verbal. Since you mention you are going for a Ph.D program, I would at least consider taking the GRE instead, but regardless, I would cancel the (very low) 620 score. It won't serve you if you're looking to get into a top school.