How Can I Use Functions in PHP Pages with Forms?

[Ybengepgjagrm]

I have a html form on my php webpage. There are four input box. And I'm adding new data with these. I want to do that, when I write something in input box1, input box2 will be same but with "-" instead of a space.. For example when I write "how are you" to input box1, input box2 must be "how-are-you".. How can I do that? :/