在android中使用json调用php函数时，响应为null

我正在尝试在magento中对android应用程序的用户进行身份验证。但是响应为null。 在php方面：

<?php
require_once('http://carinopharmacy.com/Mage.php'); //Path to Magento    

umask(0); Mage::app();

$id = 1;

try{ $result = Mage::getModel('customer/customer')->setWebsiteId($id)->authenticate($email, $password); }catch( Exception $e ){ $result = false; }
?>

---

什么是

$email，$password

？它们是在哪里定义的？这是一个内置的函数，具有参数email和password。脚本中

$email，$password

的值是什么？没有赋值。我不是php人。是否有必要定义变量？在调用该方法时不是替换了值。这是编程基础。如果未定义变量，如何获取其值？

    @Override
protected Void doInBackground(Void... params) {
    // TODO Auto-generated method stub
    JSONObject request=new JSONObject();
    JSONObject json_response=null;
    HttpClient client=new DefaultHttpClient();
    HttpPost httppost=new HttpPost(Login_url);
    try
    {
    request.put("email", email_text);
    request.put("password", password_text);

    HttpResponse response;
    StringEntity entity=new StringEntity(request.toString());
    httppost.addHeader("content-type","application/json");  
    response=client.execute(httppost);      
    String response_string=EntityUtils.toString(response.getEntity(),HTTP.UTF_8);   
    if (response_string.equals("true")) {
        startActivity(new Intent(Login_activity.this, Main_activity.class));
    }
    else {
        runOnUiThread(new Runnable() {

            @Override
            public void run() {
                // TODO Auto-generated method stub
                Toast.makeText(Login_activity.this, "Email and password do not match", Toast.LENGTH_LONG).show();
            }
        });
    }
    }
    catch(Exception e)
    {
        e.printStackTrace();
    }

    return null;
}