PHP User Giriş Kayıt Sistemi Nasıl Yapılır? [YARDIM]

$database_server... $database_user... $database_password... $database_name...

$database_connection = new mysqli($database_server, $database_user, $database_password, $database_name);
mysqli_query($database_connection, "SET CHARACTER SET 'utf8'");

$query = "SELECT `....` FROM `....` WHERE ....='$....'";
$result = $database_connection -> query($query);

if(isset($_POST['actionLogin'])){
        $user = strip_tags(trim($_POST['user']));
        $password = sha1(md5(sha1(strip_tags(trim($_POST['password'])))));
        if(empty($user) || empty($password)){
            echo "empty";
        }else{
            if(ifMail($user)){ /* mail kontrol fonksiyonu yaz */
                $dbh->where("user_email", $user);
            }else{
                $dbh->where("user_username", $user);
            }
            $userRow = $dbh->getOne("user");
            if($dbh->has("user")){
                $userRow = $dbh->getOne("user");
                if($password == $userRow['user_password']){
                    $_SESSION['session'] = true;
                    $_SESSION['user'] = $user;
                    echo "success";
                }else{
                    echo "false";
                }
            }else{
                echo "not_exists";
            }
        }
    }