WordPressサイトでPHPを使用して独自の目次を作成する方法

// 目次パーツ追加(※設置するディレクトリを指定してください。)
require_once('class-toc.php');

function custom_toc() {
    $index_adder = new Toc_Adder();
    $index_adder->generate_toc();
}

class Toc_Adder {

    private $index_content = '';

    public function __construct() {
        add_filter('the_content', array($this, 'add_index'));
    }

    public function add_index($content) {
        if (is_single()) {
            $pattern = '/<h[2-6][^>]*>(.+?)<\/h[2-6]>/s';
            preg_match_all($pattern, $content, $elements, PREG_SET_ORDER);

            if (count($elements) >= 1) {
                $i            = 0;
                $id           = 'chapter-';

                foreach ($elements as $element) {
                    $id           .= $i + 1;
                    $regex         = '/' . preg_quote($element[0], '/') . '/su';
                    $replace_title = preg_replace('/<(h[2-6])(.*?)>(.+?)<\/(h[2-6])>/s', '<$1$2 id="' . $id . '">$3</$4>', $element[0], 1);
                    $content       = preg_replace($regex, $replace_title, $content, 1);
                    $i++;
                    $id = 'chapter-';
                }
                return $content;
            } else {
                return $content;
            }
        }
    }

    public function generate_toc() {
        ob_start();
        the_content();
        $content = ob_get_clean();
        if (is_single()) {
            $pattern = '/<h[2-6][^>]*>(.+?)<\/h[2-6]>/s';
            preg_match_all($pattern, $content, $elements, PREG_SET_ORDER);
        }
        if (count($elements) >= 1) {
            $i            = 0;
            $id           = 'chapter-';
            $toc          = '';
            $currentlevel = 0;

            foreach ($elements as $element) {
                $id           .= $i + 1;
                $regex         = '/' . preg_quote($element[0], '/') . '/su';
                $replace_title = preg_replace('/<(h[2-6])(.*?)>(.+?)<\/(h[2-6])>/s', '<$1$2 id="' . $id . '">$3</$4>', $element[0], 1);
                $content       = preg_replace($regex, $replace_title, $content, 1);

                if (strpos($element[0], '<h2') !== false) {
                    $level = 1;
                    $item_class = 'heading-2';
                } elseif (strpos($element[0], '<h3') !== false) {
                    $level = 2;
                    $item_class = 'heading-3';
                } elseif (strpos($element[0], '<h4') !== false) {
                    $level = 3;
                    $item_class = 'heading-4';
                } elseif (strpos($element[0], '<h5') !== false) {
                    $level = 4;
                    $item_class = 'heading-5';
                } elseif (strpos($element[0], '<h6') !== false) {
                    $level = 5;
                    $item_class = 'heading-6';
                }

                while ($currentlevel < $level) {
                    if (0 === $currentlevel) {
                        $toc .= '<ol class="index__list">';
                    } else {
                        $toc .= '<ol class="index__list_child">';
                    }
                    $currentlevel++;
                }

                while ($currentlevel > $level) {
                    $toc .= '</li></ol>';
                    $currentlevel--;
                }
                $toc .= '<li class="index__item"><a href="#' . $id . '" class="index__link ' . $item_class . '">' . $element[1] . '</a>';
                $i++;
                $id = 'chapter-';
            }
            while ($currentlevel > 0) {
                $toc .= '</li></ol>';
                $currentlevel--;
            }

            $this->index_content = '<div class="toc-design"><div class="toc_title"><span class="toc_title_icon"><svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 24 24"><path d="M3.8 17.2h-3c-.5 0-.8.4-.8.8v3c0 .4.3.8.8.8h3c.4 0 .8-.3.8-.8v-3c-.1-.4-.4-.8-.8-.8zm0-15h-3c-.5 0-.8.4-.8.8v3c0 .4.3.8.8.8h3c.4 0 .8-.3.8-.8V3c-.1-.4-.4-.8-.8-.8zm0 7.6h-3c-.5 0-.8.3-.8.7v3c0 .4.3.8.8.8h3c.4 0 .8-.3.8-.8v-3c-.1-.4-.4-.7-.8-.7zM23.2 18h-15c-.4 0-.8.3-.8.8v1.5c0 .4.3.8.8.8h15c.4 0 .8-.3.8-.8v-1.5c0-.5-.3-.8-.8-.8zm0-15h-15c-.4 0-.7.3-.7.8v1.5c0 .4.3.7.7.7h15c.5 0 .8-.3.8-.8V3.8c0-.5-.3-.8-.8-.8zm0 7.5h-15c-.4 0-.8.3-.8.8v1.5c0 .4.3.8.8.8h15c.4 0 .8-.3.8-.8v-1.5c0-.5-.3-.8-.8-.8z" /></svg></span><span class="toc_title_text">目次</span><span class="toc_title_open_btn" id="toc_open_btn">表示</span></div><div class="toc_list">' . $toc . '</div></div><script>window.addEventListener("DOMContentLoaded",()=>{CustomTOCEvent()});const CustomTOCEvent=()=>{let t=document.querySelector(".toc-design");if(!t)return!1;let e=t.querySelector("#toc_open_btn");e.addEventListener("click",n=>{n.preventDefault(),t.classList.toggle("show"),t.classList.contains("show")?e.textContent="非表示":e.textContent="表示"})};</script>';
            $this->insert_toc();
        }
    }

    public function insert_toc() {
        ob_start();
        echo $this->index_content;
        echo ob_get_clean();
    }
}

<?php custom_toc(); ?>