<form name="nomeform" action="nomediquestapagina.php" method="get">



<select name="luca" onChange="document.nomeform.submit()">
<option value="1">1</option>
<option value="2">2</option>
<option value="3">3</option>
</select>
</p>


valore: <? print $_GET['luca']; ?></p>
</form>

ecco

le cose colorate sono da cambiare