Dear Professor ***,
I am please to learn that I have been admitted by your PHD program and awarded an assistant. I am writing this letter to inform you that I would accept the enrollment and financial assistant.
During my application for your program, I got a lot of help and advice from you. Now, I would like to thank you for all the consideration you gave me in this procedure. I hope to see you soon on campus in August. If some other documentation is needed before I register, please write to me or contact me by email. Again, thank you for your help.