Redirriger l'utilisateur ver sa session

<?php ;?>
<?php
 session_start();
$user = "cisselisan";
$password_admi = '12345678';
 if (isset($_POST['submit'])) {

$username = $_POST['username'];

$password = $_POST['password'];

if ($username&&$password) {
  # code...
  if ($username==$user&&$password==$password_admi) {
   # code...

$_SISSION['username'] = $username;
   header('location: admin/admin.php');

}
 else{
  echo "identifient erone";
 }
}else
{
   echo "veiller remplir les champs";
  }
 }

?>
<form action="" method="POST">
     <h3>Pseudo</h3><input type="text" name="username" id="username" placeholder="pseudo" ></br>
     <h3>mots-de-passe</h3><input type="password" name="password" id="password" placeholder="mots-de-passe" ></br>
     <input type="submit" name="submit" id="submit" value="valider"></br>
  </form>