Pour quoi ce code affiche toujours la session de l'enseignant ?

Bonjour,

pour quoi ce code affiche toujours la session de l'enseignant ?


<?php
if(isset($_POST["user"]) && isset($_POST["passe"]))
{
mysql_connect("localhost","root","");
mysql_select_db("encadrement");
$req = "SELECT nom FROM utilisateur WHERE user='".$_POST["user"]."' AND passe='".$_POST["passe"]."'";
$res = mysql_query($req);
if($ligne=mysql_fetch_array($res))
{
session_start();
$_SESSION["user"]=$_POST["user"];
$_SESSION["passe"]=$_POST["passe"];
$_SESSION["nom"]=$ligne["nom"];
if($user=="etudiant")
{
header("location:etudiant.php");
}
else if($user=="secritaire")
{
header("location:secritaire.php");
}
else
{
header("location:enseignant.php");
}}
else
{
?>
<form method="post" style="margin-top:20px; margin-left:10px;">
<fieldset style="width:260px; margin-top:10px;">
<legend><strong>Authentification</strong></legend>
<table width="300" border="0" align="center">
<tr>
<td width="81">Identifiant :</td>
<td width="52"><input type="text" name="user" /></td>
</tr>
<tr>
<td>Mot de passe :</td>
<td><input type="password" name="passe" /></td>
</tr>
<tr>
<td></td>
<td>
<input type="submit" name="conn" value=" OK">
</td>
</tr>
</table>
</fieldset>
<?php
echo("Erreur d'authentification");
}
}
else
{
?>
<form method="post" style="margin-top:20px; margin-left:10px;">
<fieldset style="width:260px; margin-top:10px;">
<legend><strong>Authentification</strong></legend>
<table width="300" border="0" align="center">
<tr>
<td width="81">Identifiant :</td>
<td width="52"><input type="text" name="user" /></td>
</tr>
<tr>
<td>Mot de passe :</td>
<td><input type="password" name="passe" /></td>
</tr>
<tr>
<td></td>
<td>
<input type="submit" name="conn" value=" OK">
</td>
</tr>
</table>
</fieldset>
<?php
}
?>