I intend to apply to European Master's programs in theoretical physics, and I've got my bachelor's degree in my country Thailand.

I also would like to continue my Ph.D. on my master's thesis subject, with the same professor, of course after finishing the Master's.

How could I make sure before applying for masters which of the professors would certainly accept their own Master's students to be their Ph.D. students, as well? Under which conditions?

I should mention that some people saying the European professors only accept their own masters students as their Ph.D students,and no one else.So,please also let me know if that was true.Thanks,