wordpress-ajax-taxonomy-search.php

<?php
/*
Plugin Name: Ajax Taxonomy
Version: 1.0
 */

wp_enqueue_script('jquery');

add_action('wp_ajax_my_prefix_ajax_taxonomy', 'my_prefix_ajax_taxonomy');
add_action('wp_ajax_nopriv_my_prefix_ajax_taxonomy', 'my_prefix_ajax_taxonomy');

function my_prefix_ajax_taxonomy() {

    $result = [];

    $args = array(
        'taxonomy'      => array( 'post_tag' ), // taxonomy name
        'orderby'       => 'id',
        'order'         => 'ASC',
        'hide_empty'    => false,
        'fields'        => 'all',
        'name__like'    => $_GET['q']
    );

    $terms = get_terms( $args );

    $count = count($terms);

    if($count > 0){
        foreach ($terms as $term) {
            $result[] = [
                'id' => $term->term_id,
                'text' => $term->name
            ];
        }
    }


    echo json_encode($result);

    die();
}

function my_prefix_ajax_select_shortcode() {
    $html = '
        <script src="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.13/js/select2.min.js" integrity="sha512-2ImtlRlf2VVmiGZsjm9bEyhjGW4dU7B6TNwh/hx/iSByxNENtj3WVE6o/9Lj4TJeVXPi4bnOIMXFIJJAeufa0A==" crossorigin="anonymous"></script>
        <link rel="stylesheet" href="https://cdnjs.cloudflare.com/ajax/libs/select2/4.0.13/css/select2.min.css" integrity="sha512-nMNlpuaDPrqlEls3IX/Q56H36qvBASwb3ipuo3MxeWbsQB1881ox0cRv7UPTgBlriqoynt35KjEwgGUeUXIPnw==" crossorigin="anonymous" />
        
        
        <script >
            jQuery(document).ready(function($) {
                $(".js-data-example-ajax").select2({
                    width: 500,
                    theme: "classic",
                    minimumInputLength: 3,
                    placeholder: "Search ...",
                    ajax: {
                        url: "'.admin_url('admin-ajax.php').'",
                        dataType: "json",
                        data: function (params) {
                            return {
                                q: params.term,
                                action: "my_prefix_ajax_taxonomy"
                            };
                        },
                        processResults: function (data) {
                            return {
                                results: data
                            };
                        }
                    }
                });
            });
        </script>
        
        <form>
            <select onchange="this.form.submit()" name="tax" class="js-data-example-ajax"></select>
        </form>
    ';

    // display result when user searched
    if(!empty($_GET['tax'])) {
        $html .= '<p>You searched for: '.$_GET['tax'].'</p>';
    }

    return $html;
}
add_shortcode('ajax_select_form', 'my_prefix_ajax_select_shortcode');