hope this helps
imagine you have a mysql query (lets imagine you are selecting the row where ID is 19, from the table "table", and it has the value YES, in field2, where YES or NO indicates if it is checked or not.
$query = mysql_query("SELECT * FROM table WHERE id=". $_GET['id']);
$checkbox_output = mysql_result($query, 0, 'field2');
if ($checkbox_output=='YES')
{
$checkbox=' checked';
}
elseif ($checkbox_output=='NO')
{
$checkbox='';
}
then, your html code should go like this:
<input name="checkbox" type="checkbox" value="checkbox"<? echo $checkbox; ?>>
hope this helps!