无法返回正确的json响应android

我已经把php脚本放在服务器上了。当我将该脚本与表单一起用于测试时，它运行良好。我已经在我的android应用程序中添加了php脚本所在的地址，现在当我执行代码时，它不会返回json数据，而是返回这个。

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以下是php脚本：

<?php
require("config.inc.php");
if (!empty($_POST)) {
$query = " 
        SELECT 
            id, 
            username, 
            password
        FROM userCredentials 
        WHERE 
            username = :username 
    ";
$query_params = array(
    ':username' => $_POST['username']
);
try {
    $stmt   = $db->prepare($query);
    $result = $stmt->execute($query_params);
}
catch (PDOException $ex) {

    $response["success"] = 0;
    $response["message"] = "Database Error1. Please Try Again!";
    die(json_encode($response));
}
$validated_info = false;

$row = $stmt->fetch();
if ($row) {
    if ($_POST['password'] === $row['password']) {
        $login_ok = true;
    }
}
if ($login_ok) {
    $response["success"] = 1;
    $response["message"] = "Login successful!";
    die(json_encode($response));
} else {
    $response["success"] = 0;
    $response["message"] = "Invalid Credentials!";
    die(json_encode($response));
}
 }
?>

配置inc.php

<?php 
$host = "db.freewebhost.com"; 
$dbname = "hello";     
$username = "android"; 
$password = "12345"; 

$options = array(PDO::MYSQL_ATTR_INIT_COMMAND => 'SET NAMES utf8'); 
try 
{ 
    $db = new PDO("mysql:host={$host};dbname={$dbname};charset=utf8", $username, $password, $options); 
} 
catch(PDOException $ex) 
{ 
    die("Failed to connect to the database: " . $ex->getMessage()); 
} 
$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION); 
$db->setAttribute(PDO::ATTR_DEFAULT_FETCH_MODE, PDO::FETCH_ASSOC); 
if(function_exists('get_magic_quotes_gpc') && get_magic_quotes_gpc()) 
{ 
    function undo_magic_quotes_gpc(&$array) 
    { 
        foreach($array as &$value) 
        { 
            if(is_array($value)) 
            { 
                undo_magic_quotes_gpc($value); 
            } 
            else 
            { 
                $value = stripslashes($value); 
            } 
        } 
    } 
    undo_magic_quotes_gpc($_POST); 
    undo_magic_quotes_gpc($_GET); 
    undo_magic_quotes_gpc($_COOKIE); 
} 
header('Content-Type: text/html; charset=utf-8'); 

session_start(); 
?>

为什么这个脚本不返回json数据？请帮我解决这个问题。我没有PHP方面的经验。

问候