the scripting engine does not set $doit when i submit

<input type="image" name="do" src="http://www.xxx.com/buttons/fund_off.jpg">

sample code to test:

<? if (isset($doit)) print "yes";
   else print "no";
?>
<form name="form1" method="post" action="./test.php">
<input type="image" name="doit"    src="http://www.xxx.com/buttons/fund_off.jpg">
</form>

when i click on the image, i still get a "no"