Aggiungo un pezzo di codice per darvi un'idea più chiara della situazione:

<tr><td>Nome*</td>
<form action="index.php?pagina=1" method="post">
<td>

<select name="nome" size="1" onchange="">
<OPTION VALUE="SCEGLI">SCEGLI</OPTION>
<OPTION VALUE="SCEGLI"></OPTION>
<?$query1="select * from nome ";
$res1=mysql_query($query1);
$num1=mysql_numrows($res1);

for($i=0;$i<$num1;$i++)
{
$nome_id=mysql_result($res1,$i,"prin_id");
$nome_pers=mysql_result($res1,$i,"nome_pers");
?>
<option <?if ($nome_id == $nome) echo "selected";?> value='<?echo $prin_id."'> ".$nome_pers." </option>";
}
?>
<input type='hidden' value='<? echo $nome_id; ?>' name='h_nome'>

</select>
</td></tr>
<tr><td>telefoni</td>
<form action="index.php?pagina=1" method="post">
<td>
<?
if($nome=='selected')
$query2="select * from telefoni and telefoni='h_nome'";
else
$query2="select * from telefoni";
?>

<select name="tel" size="1">
<OPTION VALUE="SCEGLI">SCEGLI</OPTION>
<OPTION VALUE="SCEGLI"></OPTION>
<?

$res2=mysql_query($query2);
$num2=mysql_numrows($res2);

for($i=0;$i<$num2;$i++)
{
$tel_id=mysql_result($res2,$i,"tel_id");
$telefono=mysql_result($res2,$i,"telefono");
?>
<option <?if ($tel_id == $tel) echo "selected";?> value='<?echo $tel_id."'> ".$telefono." </option>";
}
?>
<input type='hidden' value='<? echo $tel_id; ?>' name='h_tel'>
</select>
</td></tr>