How to destroy a $variable

Ok I have a form with a $PHP_SELF target, and I enter parts of the form when they enter information if something is wrong with their input. Now what I would like to do is destroy all the variables when the form is correct and posted, so that when the page comes back up again, I post a thank you message, and the form is blank. Just a thought. I tried setting strings to empty after data is submitted to the database like so $Age = ""; etc.... at the bottom of the file. Bounce some ideas of me. Thanks