Running a PHP command from a web page link?

I want to make a web page which controls a process on the server itself. I'd like to have it be that there's a link on the web page, and when you click that link, it runs some PHP code which executes a command inside `backticks`.

I don't want to have to reload the web page or open another window, but I don't know of any other way to execute a bit of PHP code in reaction to a link being clicked.

Can anyone help me think of a way to do this? Thanks!