You can purchase a smart lock that you control with your phone easily enough, but it's much more fun to actually make one yourself. YouTubers Hacker House make one with a Raspberry Pi.
The idea here is to power a simple lock mechanism with a Raspberry Pi, a high torque servo, and some 3D printed parts. The smartphone software is handled by Blynk. The end result isn't pretty, but it certainly does the job and you can lock or unlock the door from your smartphone. Head over to YouTube for the full parts list and code.
How to Make a Smartphone Connected Door Lock [Hacker House]