back button

hi, i´m doing a mysql-win2000-apache-php application and i wanna put a funtion (in php) to be fired in my definde back button, but dammit that blasted funciont works... too much... every time i load up the php with that button the function is fired even if the button is not pressed... any ideas to how i can put that funcion to be fired only when the user press the button? (i cant use the form commands cause i need the page accessed by the back button Cassiano