PHP TOTAL

Rebonsoir =D: voila mon code php panier:

<?php
session_start();
//panier.php

if ($_GET['action']=="destroy"){
session_unset();
session_destroy();
}
$ref = $_POST['choix'];
$qte = $_POST['qte'];

$_SESSION['achats'][] = array('ref' => $_POST['choix'],'qte' => $_POST['qte']);
echo "<br>";

$i = 0;
echo "<table border=1 cellspacing=0 cellpadding=0 bordercolor=black>";
echo "<tr>";
echo "<td>Produit</td>";
echo "<td>Description</td>";
echo "<td>Code Article </td>";
echo "<td>Dispo.</td>";
echo "<td>Prix unitaire</td>";
echo "<td>Qté</td>";
echo "</tr>";
do{
echo "<tr>";
echo "<td>".$_SESSION['achats'][$i]['ref']."</td>";
echo "<td>".$_SESSION['achats'][$i]['qte']."</td>";
echo "</tr>";
$i++;
}while (isset($_SESSION['achats'][$i]));
echo "</table>";

echo "<a href=panier.php?action=destroy>Détruire les sessions</a>";
echo "<br><a href=formulaire.htm>Retour</a>"
?>


Je voudrais que celui ci calcul le montant total des frais.
Merci;