checkbox objects returning false for isset() when submitted via a form.

Hi, I have a form that submits to itself via: <form name="hsform" method="POST" action="<?=$_SERVER['PHP_SELF'] ?>" > Within my form, I have some checkbox objects. If I check them and submit my form, isset(variable) returns true. However, if I do not check them and submit my form, isset(variable) returns false. I am confused as I though that isset() returns true if the variable exists regardless if it is empty or not. Is there a way to do what I want? Using php 4.2.1 on RedHat Linux 6.2 Thanks, Don