AJAX调用在chrome扩展中未成功

我正试图从服务器上的一个目录中获取图像的数量。我使用PHP来计算图像的数量，使用AJAX（在我的chrome扩展中）从我的外部PHP文件中获取图像的数量。

它正在执行成功：函数｛｝，但没有运行if（data）｛｝。它偶尔也会提醒错误，说发生了以下错误：error。我该如何解决这个问题？

main.js

...

(function ($) {
       $(document).ready(function(e) {
           $.ajax({
               url: 'http://website.com/numberOfImages.php',     // <== POINTS TO THE VALID URL OF THE PHP FILE THAT OUTPUTS THE NUMBER OF IMAGES...
               dataType: "HTML",
               cache: false,
               type: "POST",
               //HANDLE THE SUCCESS CASE FOR THE AJAX CALL
               success: function (data, textStatus, jqXHR) {
                   if(data){
                       alert(data);
                   }
               },
               //HANDLE THE FAILURE CASE FOR THE AJAX CALL
               error: function (jqXHR, textStatus, errorThrown) {
                   alert('The following error occurred: ' + textStatus, errorThrown);
               },
               //HANDLE THE EVENT-COMPLETE CASE FOR THE AJAX CALL
               complete: function (jqXHR, textStatus) {
               }
           });
       });
   })(jQuery);

manifest.json

{
  "name": "some name",
  "description": "Add description",
  "version": "1.0",
  "permissions": [
    "activeTab"
  ],
  "chrome_url_overrides" : {
    "newtab": "main.html"
  },
  "background": {
    "scripts": ["jquery-2.2.3.min.js", "background.js"],
    "persistent": false
  },
  "permissions": [
    "activeTab",
    "https://ajax.googleapis.com/",
    "storage",
    "tabs",
    "http://*/*",
    "https://*/*"
  ],
  "browser_action": {
    "default_title": "some title"
  },
  "manifest_version": 2
}

numberOfImages.php

<?php
    $dir        = "/home/some/directory";
    $fi         = new FilesystemIterator($dir, FilesystemIterator::SKIP_DOTS);
    $fileCount  = iterator_count($fi);
    die($fileCount);
?>