If you are talking about a linear PID controller, then there is an already defined block in Control System Toolbox in Simulink. I could be more helpful if you can be more specific.
Ok, in Simulink you can generate code for your PID controller using Embedded Coder(there are other coders but this is the most used one). Then open code generation report and see how the PID controller is implemented. The generated code is in C language, which is really easy to understand.
Build a simple model, generate code for it and see how PID is implemented.
You can also select a web site from the following list:
How to Get Best Site Performance
Select the China site (in Chinese or English) for best site performance. Other MathWorks country sites are not optimized for visits from your location.