AJAX响应无法使用PHP文件

Question

我正在使用AJAX和PHP进行注册页面。

如果注册成功，signup.php将：

echo "success";

以下AJAX代码处理：

if(ajax.responseText != "success"){//if the php file does not echo "success"
    document.getElementById('status').innerHTML = ajax.responseText;//AJAX response is displayed in a div with id "status"

    } else {//if signup is successful
      window.scrollTo(0,0);
      document.getElementById("signupform").innerHTML = "OK. Check your email inbox and junk mail box in a moment to complete the sign up process by activating your account.";
    }
}

这是我的问题：   ajax.responseText //的值为“success”但是（ajax.responseText！=“success”）返回true。