codice:
<form id="form1" name="form1" method="post" action="">

	<select name="categoria" id="categoria" onchange="document.getElementById(idcat).value=this.value;">
		<option>scegli</option>
		<option value="1">cat1</option>
		<option value="2">cat2</option>
		<option value="3">cat3</option>
	</select>
	
	<input type="text" name="idcat" id="idcat" value="" />
	
</form>