<?php
session_start();
ob_start();
print_r($_SESSION);

?>

<a href="/uu/test.php"> sign in </a>

<?php

if(isset($_POST['username'])){

    $usr= $_POST['username'];
    $_SESSION['usr']= $usr;
    header('Location:/uu');

}

?>
<form action="" method="post">

Username: <br>
<input type="text" name="username"> <br>
Password: <br>
<input type="Password" name="password"> <br>
<button type="submit">Sign In</button>

</form>