title: Calling php file on wordpress page
body: <p>I have a php file which contains functions that connects with API. Functions in that file work correctly when tested from another php file. Website that should call this functions is written using wordpress. Since I'm not familiar with wordpress, how could I call them?
  For example:
  Website have login form which contains one button. On button click I also have to call my function which is implemented in external (self written) php file. 
  I do know that button click is on client side and php is on server side. What bugs me is that when I saw source code of website I've seen that all pages are written in php but I can't navigate through them.
  What would be the easyest way to call this function on those button clicks?</p>