Try this
<?php
if (isset($valor))
{
$num_elements=count($valor);
for ($i=0; $i < $num_elements; $i++)
{
echo ("Number " . $valor[$i] . " was selected.<BR>\n");
}
}
?>
This does mean you have to use valor[] as the variable name though.
Try referencing document.formulario.valor%5B%5D
This might work for what you want to do, but I am not sure. %5B and %5D are the [ and ] characters.
Hope this helps.
Dave