问题出在您的PHP代码中。试试这个：

<?php
$data = file_get_contents('php://input');
$json_data = json_decode($data , true);
if ($_SERVER['REQUEST_METHOD'] === 'POST') 
{
//code to process data
if ($data == "" || empty($json_data['user_id']) || empty($json_data['password']))
{
    $response = array('status' => false, 'message' => 'Invalid Values');    
}
else
{
    $response = array('status' => true,'message' => 'success');
}

echo json_encode($response);
}
?>