login logout page with database

 Login.php

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 <?php                    include 'connection.php';                      if(isset($_POST['login'])){$username=$_POST["username"];   $pass=$_POST["pass"];      $selquery=mysql_query("SELECT username ,password,id 

FROM registration WHERE username='$username' AND password='$pass'");         $fetch_data=mysql_fetch_array($selquery);                 $n=mysql_num_rows($selquery);         echo $n;         if($n==1)         {
             $_SESSION["username"]=$fetch_data["username"];             $_SESSION["id"]=$fetch_data["id"];                           header("Location:ordersummery.php");             die();                   }}

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logout.php

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<?php         session_start();
      unset($_SESSION['username']);        header("Location:index.php");

?>