Sample WEB API for C# (Backend, Winform) + Mysql + Php (website front end)

Am building a C# App for the back-end, of a PHP site. Not the database will be hosted on the webhost for the site. From my understanding us that I need a WEB-API. I have zero clue but some sample code would come in hand.

Plus advise as I have started developing the C# WinForm and its performing the required DB manipulations. Not sure if Implementing a WEB-API will affect my code(assuming i might be reuired to re-write most db maniplation code since i dont use SP(stored procedrue))Gravell