I have some pages in website that i want to protect with php sessions so only an administrator with a valid password and login that match password and login in a mysql database can have access to this pages . here's the code for index.html ( the form of authentification )

<form id="form2" name="form2" method="post" action="authagent.php">
<p class="kkm">Authentification    </p>
<table align="center" width="300" border="0">
  <tr>
    <td width="146">Login</td>
    <td width="144"><label for="textfield12"></label>
    <input type="text" name="login" id="text" /></td>
  </tr>
  <tr>
    <td width="146">Mot de passe</td>
    <td><label for="textfield13"></label>
    <input type="password" name="mdp" id="mdp" /></td>
  </tr>
  <tr>
    <td>&nbsp;</td><td><input type="submit" name="button" id="button" value="Se connecter" /></td>

  </tr>

</table>
<p align="center"><a href="ajoutagent.html">Créer un nouveau compte</a></p>
<p align="center"><a href = "javascript:history.back()">

and this is the code of authagent.php

       <?php
session_start() ;
$_SESSION['connect']=0;
mysql_connect("localhost","root","") or die(mysql_error());
mysql_select_db("agence");
$login = $_POST['login'];
$mdp = $_POST['mdp'] ;

$query = "SELECT * FROM agent where login_agent = '$login' and mdp_agent = '$mdp'";
$result = mysql_query($query);
while ($line = mysql_fetch_array($result, MYSQL_ASSOC)) {


if ($login == $line['login_agent'] && ($mdp == $line['mdp_agent'])) // Si le nom d'utilisateur et le mot de passe sont correct
{
 $_SESSION['connect']=1; 
          header('Location: agent.php');   

}
else
{
echo 'incorrect' ;// Si le nom d'utilisateur ou le mot de passe est incorrect
}
}
?>

Here's the code of a secured page agent.php

 <?php
session_start();
if (isset($_SESSION['connect']))//On vérifie que le variable existe.
{
        $connect=$_SESSION['connect'];//On récupère la valeur de la variable de session.
}
else
{
        $connect=0;//Si $_SESSION['connect'] n'existe pas, on donne la valeur "0".
}

if ($connect == "1") // Si le visiteur s'est identifié.
{
              header('Location: agent.php');  

// On affiche la page cachée.
}
else
{
                  header('Location: seconnecteragent.php');  


    } ?>

• php
• html

Usually this is done by testing for the existence of a session variable like loggedin, and if it is not =1 then you automatically redirect to the login page. You can put this simple bit of code at the top of every page, and if the loggedin variable is there, nothing happens and the page is served normally. A basic example:

<?php 
if(!isset($_SESSION['loggedin']) || $_SESSION['loggedin']!=1){
    header('Location: login.php');
    exit();
}
?>

As I can see, your problem is that you have a recursion there. In agent.php page, if the user is authenticated, then you send him back to the same page agent.php.