我創建了一個注冊和登錄系統！注冊沒有問題。我輸入所需的用戶名，所需的電子郵件，所需的密碼并按回車鍵，點擊回車鍵后，我來到主頁。問題是當我按下注銷按鈕時，當離開按鈕將我帶到登錄頁面時，當我重新輸入用戶名和密碼時，我得到“不正確的用戶名或密碼”錯誤。   if (isset($_POST['register'])) {        $username = mysqli_real_escape_string($db, $_POST['username']);        $email = mysqli_real_escape_string($db, $_POST['email']);        $password = mysqli_real_escape_string($db, $_POST['password']);        $repeatpassword = mysqli_real_escape_string($db, $_POST['repeatpassword']);        $username_checker = "SELECT * FROM users WHERE username='$username'";        $email_checker = "SELECT * FROM users WHERE email='$email'";        $name_checker = mysqli_query($db,$username_checker) or die(mysqli_error($db));        $mail_checker = mysqli_query($db,$email_checker) or die(mysqli_error($db));        if(empty($username)){            array_push($errors,"Username is required");            return;        }        if(empty($email)){            array_push($errors,"Email is required");            return;        }        if(empty($password)){            array_push($errors,"Password is required");            return;        }        if(mysqli_num_rows($name_checker) > 0){            array_push($errors,"Username is already recorded in our database");            return;        }        if(mysqli_num_rows($mail_checker) > 0){            array_push($errors,"Email Address is already recorded in our database");            return;        }        if(!preg_match("/^[a-zA-Z ]*$/",$username)){            array_push($errors,"The username is only derived from uppercase and lowercase characters");            return;        }        if(strlen($_POST['username']) < 5){            array_push($errors,"Username must be at least 5 characters long");            return;        }