This is a nice build where the author uses an arduino like board with NFC built in to control a servo that locks/unlocks the door. The downside, the NFC board is $40. Bluetooth maybe a better…well cheaper…solution. The other thing I do not like is there is no manual override.
http://www.instructables.com/id/NFC-Door-Lock-with-the-Qduino-Mini-under-100/?ALLSTEPS