if you do a master's dissertation it normally ends up being the first few chapters of your phD thesis.
However, a PhD is completely unlike any previous kind of study. think about why you are really enjoying the study at the moment, what aspect of it thrills you? not in the subject matter but in the functions of reading and thinking and applying that thinking and perhaps measuring and evaluating etc etc.
prior to PhD study you are being tested and graded on how you can study, absorb, reproduce and think about existing knowledge in your field.
"I did a masters and really enjoyed it" is not, in my opinion, a good reason to do a PhD.
Doing a Bachelor degree gives you the skills to read and learn independently without being spoonfed facts, and can reproduce that learning reliably.
At masters level you are more focusing on how we know what we know and learning to critique and evaluate the limits of what we know. up till fairly recently a master's qualification was also a licence to teach on any subject because someone with a master's degree is supposed to have the skill to read up on and then break down into fundamentals any topic in academia. (there is a bit more to teacher training these days).
at PhD level you are supposed to be generating new knowledge and the process by which it is evaluated isn't by someone marking your work and giving you a grade, it's by academics looking for flaws and wooliness in your thinking, and if they are doing their job right, picking your work to pieces and being hyper critical if it doesn't hold together, because only work that can survive that process is fit to be added to the corpus of knowledge in your field.
if you enjoyed study at a particular level that doesn't mean you will necessarily be as happy continuing to the next level - you might be happier just applying the skills you have at the level you are at, with independent reading of any subject of interest applying the skills you have already proved that you have.