First of all : PHP and JavaScript are two entirely different things. So skip the ".checked"-syntax and test if the chkbox has a value set or not.
If a checkbox is checked, "checked" is added like this :
<input name="chkbox" type="checkbox" checked value="on">Whatever
If not, leave "checked" out.
To see if the checkbox is checked or not, look at the value. If it's "on" it's checked if not, it's unchecked :
<?php
if ($chkbox == 'on') {
echo "I'm checked!";
} else {
echo "I'm NOT checked!";
}
?>
If you need a more specific example, contact me and I'll whip up some code.