simple one file php session for login example

index.php

<?php

// valid credentials
$valid_user='user';
$valid_password='pwd';

if (isset($_POST['login']) && isset($_POST['pwd'])) {
   // trying to log
   if ( ($_POST['login'] == $valid_user) and
        ($_POST['pwd'] == $valid_password) ) {
      // login ok
      session_start();
      $_SESSION['login'] = $_POST['login'];
      $_SESSION['pwd'] = $_POST['pwd'];
      header('location:index.php');
   }
   else {
      // login ko
      exit();
   }
}

session_start();
if (isset($_SESSION['login']) && isset($_SESSION['pwd'])) {
   // session is ok
   if (isset($_POST['btn']) && $_POST['btn'] == 'unlog') {
      // deconnexion
      session_unset ();
      session_destroy ();
      header('location:index.php');
   }
   echo "<h2> Hello, ". $_SESSION['login'] ." </h2>";
   echo '<form action="index.php" method="post">
   <button name="btn" type="submit" value="unlog">Deconnexion</button>
   </form>';
} else {
   // not logged
   session_destroy();
   echo '<h2>Login</h2>';
   echo '<form action="index.php" method="post">
   login : <input type="text" name="login">
   <br />
   password : <input type="password" name="pwd"><br />
   <input type="submit" value="Connexion">
   </form>';
}

?>