Can you still be cited for curfew if you are driving a car? or is it just if you are walking and loitering?

Curfew still counts even if you are in a car.  Curfews may vary depending on where you live.  Each city has its own rules regarding curfew and its own exceptions. So you may want to look up the curfew and its rules in your city. Look at question #18006 for curfews for all Phoenix metro cities.

Also, consider the new law that took effect on July 1, 2008, A.R.S. § 28-3174. That law states that for the first six months after a person gets a class G license, that person cannot drive on a public highway between midnight and 5:00 a.m. unless they are (1) with a parent or legal guardian who has a class A, B, C, or D license and is sitting in the front seat beside the driver or (2) the driver is driving directly from a sanctioned school sponsored activity, the licensee's place of employment, a sanctioned religious activity or a family emergency.

Keep in mind that according to this law, a peace officer cannot stop or issue a citation to a person unless that officer has reasonable cause to believe that there is another violation of a motor vehicle law.