Stuck pada saat ingin menampilkan data

Saya mengikuti kelas pembuatan form login php tapi stuck pada saat ingin menampilkan data yang telah dilogin adakah yang bisa membantu saya?

//untuk login.php
<?php
    require_once 'core/init.php';

    $errors = array();

    if( Input::get('submit') ){

        //1. mengambil objek validasi
        $validation = new Validation();

        //2. metode check
        $validation = $validation->check(array(
            'username' => array( 'required' => true ),
            'password' => array( 'required' => true )
        ));

        //3. lolos ujian
        if ( $validation->passed() ) {

            if( $user->login_user( Input::get('username'), Input::get('password') ) )
            {
                Session::set('username', Input::get('username'));
                header('Location: profile.php');
            }else{
                $errors[] = 'login gagal';
            }

        }else{
            $errors = $validation->errors();
        }

    }

    require_once 'templates/header.php';
?>

<h2>Login Di Sini</h2>

<form action="login.php" method="post">
    <label>Username</label>
    <input type="text" name="username"> <br>

    <label>Password</label>
    <input type="password" name="password"> <br>

    <input type="submit" name="submit" value="login sekarang">

    <?php if(!empty($errors)){ ?>
        <div id="errors">
            <?php foreach ($errors as $error){ ?>
                <li> <?php echo $error; ?> </li>
            <?php } ?>
        </div>
    <?php } ?>

</form>

<?php require_once 'templates/footer.php'; ?>

//untuk login.php

untuk user.php
<?php

class User{

    private $_db;

    public function __construct()
    {
        $this->_db = Database::getInstance();
    }

    public function register_user($fields = array())
    {
        if( $this->_db->insert('users', $fields) ) return true;
        else return false;
    }

    public function login_user($username, $password)
    {
        $data = $this->_db->get_info('users', 'username', $username);

        if(password_verify($password, $data['password']) )
            return true;
        else return false;
    }

}

?>

//untuk user.php

//untuk database.php
<?php

class Database{

    private static $INSTANCE = null;
    private $mysqli,
            $HOST = 'localhost',
            $USER = 'root',
            $PASS = '',
            $DBNAME = 'tutorial';

    public function __construct()
    {
        $this->mysqli = new mysqli( $this->HOST, $this->USER, $this->PASS, $this->DBNAME );
        if( mysqli_connect_error() ){
            die('gagal koneksinya');
        }
    }

    /*
        singleton pattern,
        menguji koneksi agar tidak double
    */
    public static function getInstance()
    {
        if( !isset( self::$INSTANCE ) ){
            self::$INSTANCE = new Database();
        }

        return self::$INSTANCE;
    }

    public function insert($table, $fields = array())
    {
        //mengambil kolom
        $column = implode(",", array_keys($fields));

        //mengambil nilai
        $valueArray = array();
        $i = 0;
        foreach ($fields as $key=>$values){
            if( is_int($values) ){
                $valueArray[$i] = $this->escape($values) ;
            }else{
                $valueArray[$i] = "'" . $this->escape($values) . "'";
            }

            $i++;
        }
        $values = implode(", ", $valueArray);

        $query = "INSERT INTO $table ($column) VALUES ($values)";

        return $this->run_query($query, 'masalah saat memasukan data');
    }

    public function get_info($table, $column, $value)
    {
        if( !is_int($value) )
            $value = "'" . $value . "'";

        $query  = "SELECT * FROM $table WHERE $column = $value";
        $result = $this->mysqli->query($query);
        while($row = $result->fetch_assoc()) {
            return $row;
        }
    }

    public function run_query($query, $msg)
    {
        if($this->mysqli->query($query)) return true;
        else die($msg);
    }

    public function escape($name){
        return $this->mysqli->real_escape_string($name);
    }

}
?>

//untuk database.php