How to redirect to the same page with a message

hi

I am trying to use the kirupas great php tutorial to submit a contact form. But when the submit button is pressed , i want it to go to a certain HTML file (the same form file with just an additional line of ‘THANK YOU. YOUR DATA HAS BEEN SUBMITTED’ etc…).

how do i get this done in PHP. I am new to this and any help put in simple way will be highly appreciated.