How to write a sincere reply to a very positive reply from a professor?
Thank you for your reply, Professor X. I am happy to hear that you're interested in taking on a PhD student for next year, I will definitely be putting in an application for the position. I look forward to hearing more about the project in the future. Regards, me.


You don't need to throw them a parade, but take a little more time to email them something other than just, 


  Thanks a bunch!