sesi login sederhana php

Anda tidak dapat mencampur PDO dan mysql .. Anda membuat kueri di PDO dan menggunakan mysql_* Coba ubah kode Anda menjadi

<?php

// Inialize session
session_start();

// Include database connection settings
include('../../model/database.php');

// Retrieve username and password from database according to user's input
$stmt = $db->prepare("SELECT * FROM user WHERE (`username` = :username) and (`password` = :password)");

$result = $stmt->execute(array(':username'=>$_POST['username'],':password'=>$_POST['password']));
$num_rows = $stmt->rowCount();
// Check username and password match
if ( $num_rows > 0) {
// Set username session variable
$_SESSION['username'] = $_POST['username'];
// Jump to secured page
header('Location: securedpage.php');
}
else {
// Jump to login page
header('Location: index.php');
}

?>

lihat reference