我如何告诉我的php脚本运行一组方程式而不是另一组方程式

I am building calculators for my website, they are surface area type calculators and I need one metric and one for us. I really don't want to have to have 2 calculators taking up real estate on my page. Instead, I'd like to build a radio button on the top of my form and have the user click either metric or us to generate the proper code. How exactly would I go about doing this? I am just starting to get my hands in php so i apologize if this question has a simple solution.I am building calculators for my website, they