$remove = array_diff($old_cat, $new_cat);
print_r($remove);
$sql = "DELETE FROM apparel_categories WHERE apparel_id=\"{$_POST['id']}\" AND category_id=\"$remove[]\""; 
$deleted = mysql_query($sql); 
if(!$deleted)
{
	throw new Exception("While saving category (-), you got a mysql error: ".mysql_error($db)."<br>\$sql was:".$sql.'');
}


$add = array_diff($new_cat, $old_cat);
print_r($add);
$sql = "INSERT INTO apparel_categories (apparel_id, category_id) VALUES (\"{$_POST['id']}\", \"$add[]\")"; 
$added = mysql_query($sql);
if(!$added)
{
	throw new Exception("While saving category (+), you got a mysql error: ".mysql_error($db)."<br>\$sql was:".$sql.'');
}