problem with querystring

hi there, i'm having this very strange problem using a querystring... to illustrate me problem i have this file: <form method="post"> a: <input type="checkbox" value="X" <? if ($a == "X") echo "checked" ?>><br> b: <input type="checkbox" value="X" <? if ($b == "X") echo "checked" ?>><br> </form> called test.php... when i open the url tesp.php?a=X the checkbox doesn't get checked, and as far as i know some PHP it should be... i'm having this problem with every querystring i use... i'm working with release 4.1.1 on a Windows XP Prof machine... anyone has got an idea? chears, hans