Hello, a PG diploma is different from that of a masters degree in the sense that it is for a period of 1 year and has a credit of 120 while masters who credit of 180 and is of duration of 2 years.PG diploma can be done to enhance your career knowledge while you are pursuing a job and if you donot want to get into research field which is a part of masters curriculum. But you can do a PHD further only if your PG diploma is equivalent to a masters degree.

Hope this helps.