codice:
<select name="select">
<option selected value="0">Scegli</option>
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
<option value="4">4</option>
</select>



<?php
if ($_POST['select'] == 0)
    echo "Selezionare la tendina";
else echo "OK";
?>