Una cosa cosi?
Codice PHP:
<script type="text/javascript">
function changeHiddenInput (objDropDown)
{
var objHidden = document.getElementById("hiddenInput");
objHidden.value = objDropDown.value.substring(objDropDown.value.indexOf("-")+1);
}
</script>
<select id="dropdown" name="dropdown" onchange="changeHiddenInput(this)">
<option value="primo-42" onchange="set hiddenInput - 42">Uno - 42</option>
<option value="secondo-40" onchange="set hiddenInput - 40">Due - 40</option>
<option value="terzo-38" onchange="set hiddenInput - 38">Tre - 38</option>
</select>
<input type="text" name="hiddenInput" id="hiddenInput" value="" />