Use Baked-In AJAX to Query Checked Tags

<?php

/*
Plugin Name: Ajax Tags
Description: Retrieve list of posts instantly by selecting a tag.
Version: 1.0
Author: Stephanie Scharf
Author URI: http://stephscharf.me
*/


// Get all tags in use
// call list_ajax_tags() in theme where you want to display these tags
function list_ajax_tags() {

    $tags = get_tags();
    $tag_list = '<div class="post_tags">';
    foreach ($tags as $tag){
        $tag_list .= '<input class="tagcheck" type="checkbox" value="'.$tag->slug.'" />'.$tag->name;

    }
    $tag_list .= '</div>';
    $tag_list .= '<div id="tag_results"></div>';

echo $tag_list;
}


 // Load jQuery/CSS
 // Codex: http://codex.wordpress.org/AJAX_in_Plugins
add_action('wp_head', 'ajax_tags_jquery');
function ajax_tags_jquery() {
    $filepath = plugin_dir_url( __FILE__ );
    ?>

        <script type="text/javascript">
        jQuery(document).ready(function($){
          $(".tagcheck").click(function() {
                $slug = $(this).val();
                var data = {
                    action: 'tag_slug',
                    slug: $slug
                };

            var ajaxurl = 'wp-admin/admin-ajax.php';
            jQuery.post(ajaxurl, data, function(response) {
                $('#tag_results').html(response);
            });
        });
        });
        </script>

<?php }

// Invoke baked-in WP ajax goodness
// Codex: http://codex.wordpress.org/AJAX_in_Plugins
add_action('wp_ajax_tag_slug', 'tag_slug_callback');

function tag_slug_callback() {
    global $wpdb;

function taglist() {
    // Retrieve tag slug from POST
    $tagslug = $_POST['slug'];
    ?>

    <ul>
    <?php
    // Query 5 posts from the tag that was selected
    $args = array( 'posts_per_page' => 5, 'tag' => $tagslug );
    $tag_query = new WP_Query( $args );
    while ( $tag_query->have_posts() ) : $tag_query->the_post();
    ?>
        <li><a href="<?php the_permalink(); ?>"><?php the_title(); ?></a></li>

    <?php endwhile; wp_reset_postdata(); ?>
    </ul>

    <?php }

    echo taglist();

    die(); // this is required to return a proper result
}
?>