Search only in specific category

[Hexilium]

Hi there, I want to make a search form only for specific category. Well I want to add a variable in adress bar like this:

&category_id=2

But to make it work I have to make relevant query in controler? Well thats it, i know PHP only a bit and I dont know prestashop code so im asking here for help. How to change search to make it use category variable if I want to search only in specific category?

 

I gues something like this should be added hmm somewhere in controler or?

'WHERE id_category=$_POST($category_id)'

 Well, help