Checked / unchecked error messages

[qwikad.com]

On one of my pages if both checkboxes are checked this error will show up:

            if ($_POST['promote']['multicity_co'] && $_POST['promote']['multicity_cf'])
                $err .= "&bull; please, select only ONE paid promotion<br>";

My question is, what code should I use to give an error message if NONE of the checkboxes are checked?

Thank you in advance.

[bernie1227]

I assume you could use something like this:

            if (!isset($_POST['promote']['multicity_co']) && !isset($_POST['promote']['multicity_cf']))
                $err .= "&bull; please, select a paid promotion<br>";