外部php文件的500（内部服务器错误）

I am currently using WP Web Scraper version 3.2. Once I insert the shortcode or template tag (php code) directly into my page, the plugin works perfectly fine and the values appear. I decided to place the template tag in an external php file and use Ajax to call the code after a specific amount of time, and once I do so nothing appears and my console shows 500 (Internal Server Error). I get a message which says "Failed to load resource". My php file (test.php) contains this (assuming that URL is the actual link to the website):

<?php
echo wpws_get_content('URL', 'body > table > tbody > tr:nth-child(1) > td:nth-child(8)' );

On my page, I use Javascript to load this file and it looks like this:

<script src="https://ajax.googleapis.com/ajax/libs/jquery/1.11.2/jquery.min.js"></script>
<script>
$(document).ready( function() {
setInterval(function () {
    $('#demo').fadeOut('slow', function() {
        $('#demo').load('http://localhost:8888/test-site/wp-content/my-php-files/test.php', function() {
            $('#demo').fadeIn('slow');
        });
    });
}, 6000);
});
</script>

<div id="demo"></div>

I tested the PHP file with other codes to check if they work and they work perfectly fine. One example of php I inserted in that file was:

<?php
echo(rand(10,100));

In this last example, using my script, a random number is loaded from the external file every 6 seconds without having to refresh the page and I get no errors. I deduced from this the template tag provided by the plugin is causing the problem. How can I load this template tag into my script? Any suggestions?

King regards!!