This PID_controller library is documented in a blog post here: https://jowood4.wordpress.com/2015/01/04/arduino-pid-temperature-controller/

It requires a couple of library dependencies:

1. PID_v1. There is a copy in my repo, and it is based on this code: https://github.com/br3ttb/Arduino-PID-Library/
2. MAX31855. Also in my repo. This is the temp sensor I used, but your code could be adapted to use any temp sensor.