Commit

$conn = mysql_connect("localhost", "root", "");

mysql_select_db("website");

$username = $_POST['username'];
$password = $_POST['password'];

$sql = "SELECT * FROM users
        WHERE username = '$username'
        AND password = '$password'";

$result = mysql_query($sql);

if (mysql_num_rows($result) > 0) {
    $_SESSION['admin'] = true;
    header("Location: admin.php");
} else {
    echo "Wrong password lol";
}