simple yes or no checkbox for forms

I have inserted a checkbox in my form.

My code:

<input type="checkbox" id="checkbox" name="checkbox" value="1"/>

if($checkbox = ($_POST['checkbox']) == '1')
        {
            $checkbox = "si";
        }
        else
        {
            $checkbox = "no";
        }

I would like that if the checkbox is checked i receive "yes" otherwise "no". Thanks.