PHP Login dengan Username atau Email

<?php
session_start();
include "koneksi.php"; // include file koneksi database

// periksa dulu bahwa tombol login telah ditekan
if(isset($_POST['login'])){
// parsing data yang diinput user
$usermail = $mysql->escape_string(strtolower(trim($_POST['username'])));
$password = md5($_POST['password']);
if(strpos($_POST['username'],'@'))
    $field = "lower(email)=lower('$usermail')";
else
    $field = "lower(username)=lower('$usermail')";

// buat query untuk cek data
$sql = mysql_query("SELECT * from users where $field AND password='$password'");
$cek = mysql_num_rows($sql); // hitung jumlah data

// cek data, kalao ada di tabel users (login sudah benar)
if($cek>0){
    $data = mysql_fetch_array($sql); // fetch data sql
    // buat sesi
    $_SESSION['login'] = 1;
    $_SESSION['user'] = $data['username'];
    $_SESSION['nama'] = $data['nama'];
    $_SESSION['level'] = $data['level'];

    // alihkan halaman sesuai level
    if($data['level']=="admin")
        header("location:admin.php");
    else
        header("location:user.php");
}else{
    echo "<script>history.back()</script>";
}
}
?>


URL Sumber : http://www.arijulianto.com/php/studi-kasus-login-php-mysql.php#ixzz2z88s62T4