Need Checkbox to Update Message

I need you to create a checkbox that when checked displays one portion of text and when unchecked displays a different portion of text. I created my own version and it updates the link destination, but I can't figure out how to make it change the display text. I just need you to code it up so I can display two different messages - If you can get it to a point where it calls up different text in the same way the code-testing page does, then I will edit the text to fit my needs. I will give you the PHP file when I hire you.