如何在我的ajax-php脚本中访问wordpress函数

Question

I'm trying to call a WordPress function ( get_avatar($id) ) from a php script that is being accessed in an ajax referenced call from an enqueued javascript file. (Saying this out load already makes my head hurt).

As a an example, I've created a plugin for WordPress: hmm-test. It is implemented in hmm-test.php. This script enqueues a javascript file: test.js. Within the js I have an xhttprequest call that POSTS to hmm-test.php, which branches on a test for a POST variable.

When hmm-test.php is called as a GET, for example using the WordPress URL for the page where the short-code is embedded, I can call WordPress functions. From the POST branch, I want to be able to call WordPress native functions as well to return that as part of the JSON payload.

So my question is: can I make the 'POST' side branch of hmm-test.php 'wordpress-aware' such that I can call native WP functions from within it?

I'm stuck - not sure what else to try. I tried creating a REST endpoint to call via an Ajax URL early on but kept running into auth problems. If this would work, I can spend time working through it, but I've found the OAuth bits tricky. I had it working for something else, and lost it. If there's a way to do it with a simple POST, that's good enough for my proof of concept.

Here is the plugin PHP script and the enqueued javascript in their entirety:

<?php
/*
Plugin Name:  HMM Test
*/

if (strlen($_POST['action'])) {

        $rv["payload"] = get_avatar(108);
        $rv["payload"] = "This is the payload message";
        $rv["status"]  = 'SUCCESS';
        echo json_encode($rv);

} else {

        function hmm_test_enqueue_scripts() {

                wp_register_script( 'test-js', plugins_url( '/js/test.js', __FILE__ ), '', '1.0', all );

        }

        add_action( 'wp_enqueue_scripts', 'hmm_test_enqueue_scripts' );

        /* ***************************************************************************
         * This is the main page
         * ************************************************************************* */

        add_shortcode( 'test-contents', 'my_test_contents' );

        function my_test_contents() {

                wp_enqueue_script( 'test-js' );

                wp_localize_script('test-js', 'wpVariables', array(
                        'pluginsUrl' => plugins_url(),
                        'pluginDir' => trailingslashit( dirname( plugin_basename( __FILE__ ) ) ),
                        'userID' => 37
                ));

                echo "Wordpress functions test: get_avatar()";
                echo get_avatar( 37 );          # -- This works from the plugin script

                echo '<button type="button" class="test-button">Click to crash</button>';
        }
}

And the JS:

(function () {
        var c = document.getElementsByClassName('test-button');

        for (var i = 0; i < c.length; i++ ) {
                c[i].addEventListener('click', doTest, false);
        }
})();

function doTest() {

        console.log("In doTest()");

        var xhttp = new XMLHttpRequest();
        xhttp.onreadystatechange = function() {
                if (this.readyState == 4 && this.status == 200) {
                        console.log("responseText: " + this.responseText);
                }
        };

        var ajaxUrl = wpVariables.pluginsUrl + '/' + wpVariables.pluginDir + 'hmm-test.php';
        var userid  = wpVariables.userID;

        xhttp.open("POST", ajaxUrl, true);
        xhttp.setRequestHeader("Content-type", "application/x-www-form-urlencoded");
        let arg = "action=update" + "&userid=" + userid;
console.log("Calling URL:" + ajaxUrl + " with arg: " + arg);
        xhttp.send(arg);
}

When I place the shortcode [hmm-test] in a Wordpress page, it loads fine, and I get the expected 'Wordpress functions test...', the proper avatar image for user 37 (hard-coded in this example), and a button to click to make things crash (for now!).

Clicking the button, I see this is the console window:

In doTest() test.js?ver=1.0:40 Calling URL:https://not-my-real-url.com/wp-content/plugins/hmm-test/hmm-test.php with arg: action=update&userid=37 test.js?ver=1.0:30 responseText: Fatal error: Call to undefined function get_avatar() in /home4/mifydnu/public_html/wp-content/plugins/hmm-test/hmm-test.php on line 8

Obviously, and perhaps unsurprisingly, nothing is known about the get_avatar() function. This makes sense - I'm just calling some PHP script via a URL. Makes no difference that the script is sitting in a Wordpress directory somewhere. But I am stumped at how to get around this.

Thanks for any advice!