Two separate issues. Any time you are "sole manipulator of the controls of an aircraft for which you are rated" (category, class, type, and sign-offs if necessary) you may log flight time as "pilot-in-command" - part 61.51(e)(1)(i).

This doesn't necessarily mean you are PIC as defined in part 1.1. To be that person you must..."(1) has final authority and responsibility for the operation and safety of the flight; (2) has been designated as pilot in command before or during the flight; and (3) hold the appropriate category, class, and type rating, if appropriate, for the conduct of the flight". This is the pilot who owns, signs the rental agreement, signs the flight release, etc. for the flight.

If you are the sole occupant of the aircraft, you are the sole manipulator and PIC in flight by default. If you are riding along with a rated pilot buddy who rented the aircraft, but you are flying the aircraft and you are rated, then you can "log" PIC time. However he is still the PIC responsible.

So to answer the question, "can you be PIC of a 172?". Sure, get a check-out and signed off for insurance purposes, rent the airplane, and go flying (you're PIC responsible - part 1.1). While you are doing your checkout with the instructor, you can log the time you're sole manipulator of the controls as "PIC" (part 61.51), but he is still "PIC responsible".