passing variables

Hello all, I have a login/register screen for my php/mysql db. When the page opens, the user sees the log in screen but I have a button and reloads the same page, hopefully with the register screen. I know I can simply check to see if a variable is set with if ($variable){} . This does not seem to work when I give an image a name and link it to the same page. The image is named register and it links back to the same page, so the page loads, I click on the register image , the page reloads but the variable is not set. Any ideas? Thanks, Eddie