Sessions Login Logout

<?php
if(isset($_POST['sub'])){
 $uname = $_POST['txtuname'];
 $pwd = $_POST['txtpwd'];

 if($uname=="scott" and $pwd=="scott123"){
  session_start();
  $_SESSION['aut'] = true;
  echo "<script>location='welcome.php';</script>";
 }
 else{
  echo "invalid";
 }
}
?>
<form method="post" action="">
Username:<input type="text" name="txtuname">
Password:<input type="password" name="txtpwd">
<input type="submit" name="sub" value="login">
</form>

WELCOME.PHP

<?php
session_start();
if(isset($_SESSION['aut'])){
 echo "Welcome to user";
}
else{
 echo "<script>location='login.php';</script>";
}
?>
<a href="logout.php">Logout</a><link rel="canonical" href="">

LOGOUT.PHP
<?php
session_start();
session_destroy();
echo "Logout successful";
?>