Salut,
quelques petites modifs
<?php
if (isset ($_POST['Submit']) AND isset($_POST['valeur']) AND is_numeric($_POST['valeur']))
{
echo 'Valeur : ',$_POST['valeur'];
}
?>
<form action ="?" method="post">
<select name="valeur">
<option value="1">Valeur1</option>
<option value="2">Valeur2</option>
<option value="3">Valeur3</option>
</select>
<p><input type="submit" name="submit" value="Envoyer" /></p>
</form>