Hi
How do international students go to interviews?
I mean, if somedy is from Pakistan, this person should go to UK, do the interview and return to Pakistan? And if he applied for more than one PhD he risks to have to return to the UK next week, for another interview. Who pays for all this? Are there any international students with a similar experience?

I should imagine that circumstances dictate the nature of the interview process. Alot of students, myself included, are "interviewed" on the telephone which I actually prefer because of being able to have notes and a discussion guide in front of me.

Depends entirely on the institution, including whether they reimburse for travel

Phone calls and video conferencing are more common these days

I meant more used, not more common than face-to-face IYSWIM