Calling a plugin across two scripts

Hi everyone.

This is my first post – I hope I’ve asked it correctly.

I need to make a call from WordPress (using Ajax) out to a script (CALL_SHORTCODE.php below) to retrieve some data from a table. To do this, the 1st script needs to call another script (TEST_SHORTCODE.php) to get the login name of the user from a plugin.

WP (ajax) –> CALL_SHORTCODE.php –> TEST_SHORTCODE.php

I’m not getting any data back in CALL_SHORTCODE.php. However, if I just call TEST_SHORTCODE.php without calling it from CALL_SHORTCODE.php I get data I want (eh?). The following code will run.

[CALL_SHORTCODE.php]

<?php
  // Call our script to get who's logged in
  $jsonurl = "https://my.url.com/scripts/TEST_SHORTCODE.php";
  $json = file_get_contents($jsonurl);
  var_dump($json);
?>

[TEST_SHORTCODE.php]

<?php
  // Include WordPress
  // Tried using wp-blog-header.php as well to no avail
  include_once '../wp-load.php';

  // Get login name from Memberpress and return it
  $username = do_shortcode("[mepr-account-info field='user_login']");
  $rows[] = array( 'username'=>$username );
  echo json_encode($rows, JSON_NUMERIC_CHECK);
?>

…Output from calling TEST_SHORTCODE.php (valid JSON):

[{"username":"[email protected]"}]

…However, the output from calling CALL_SHORTCODE.php is:

string(17) "[{"username":""}]"

…As you can see, there is no value in the output of CALL_SHORTCODE.php.

This one is driving me bonkers ??

Many thanks in advance
Jason