WP AJAX Example class

class-wp-ajax-example.php

<?php
// WP AJAX example

class WP_AJAX_Example{

    /**
     * Add WP actions. Must be called only once.
     * @return void
     */
    public function add_actions(){

        $actions = [
            'ajax_get_clients',
            // you can add other php handler functions here
        ];

        foreach ($actions as $action) {
            add_action('wp_ajax_' . $action, $action, 10, 1);
            add_action('wp_ajax_nopriv_' . $action, $action, 10, 1);
        }

        add_action('wp_dashboard_setup', [$this, 'wp_dashboard_setup']);
    }

    /**
     * AJAX handler. Function name equals $actions[]
     *
     * @return void
     */
    public function ajax_get_clients()
    {
        $nonce = $_REQUEST['nonce'];
        $data = $_REQUEST['data'];

        if (!wp_verify_nonce($nonce, 'ajax')) {
            trigger_error('verify failed in ' . __FUNCTION__);
            wp_send_json_error();
        }

        // do something
        $result = true || false;
        if ($result) {
            wp_send_json_success($result);
        }

        wp_send_json_error($result);
    }

    /**
     * Render wp dashboard widget
     *
     * @return void
     */
    public function wp_dashboard_setup() {
        if (!current_user_can('manage_options')) {
            return;
        }
        wp_add_dashboard_widget('ajax', __('AJAX', 'wp'), [$this, 'html']);
    }

    /**
     * Render html button and add js for them.
     *
     * @return void
     */
    public function html()
    {
        ?>
        <button type="button" class="ajax button button-primary" data-action="ajax_get_clients"
                data-nonce="<?= wp_create_nonce('ajax') ?>"> AJAX button
        </button>
        <script type="application/javascript">
            jQuery(document).ready(function ($) {
                $('.ajax').on('click', function (event) {
                    event.preventDefault();
                    let $this = $(this).removeClass('success').removeClass('error').addClass('loading');
                    $.ajax({
                        type: 'POST',
                        url: '/admin-ajax.php',
                        data: {
                            'action': $(this).data('action'),
                            'data': '', // any data for php
                            'nonce': $(this).data('nonce')
                        },
                        success: function (response) {
                            console.log('ajax', response);
                            if (response.success) {
                                $this.removeClass('loading').addClass('success');
                            } else {
                                $this.removeClass('loading').addClass('error');
                            }
                        },
                    });
                });
            });
        </script>
        <?
    }
}

// run this code, if you include it as file
(new WP_AJAX_Example())->add_actions();