I’ve found the answer…
<select name="cars[]" multiple="multiple">
<option value="volvo">Volvo</option>
<option value="saab">Saab</option>
<option value="opel">Opel</option>
<option value="audi">Audi</option>
</select>
and in the PHP part :
$cars = $_POST['cars'];
print_r ($cars);