WordPress でプラグインなしで目次を自動生成する方法

if ( ! function_exists( 'custom_add_table_of_contents' ) ) {
    function custom_add_table_of_contents( $content ) {
        if ( ! is_single() ) {
            return $content;
        }
 
        $post_id     = get_the_ID();
        $toc_level   = get_post_meta( $post_id, 'toc_level', true );
        $target_level = in_array( $toc_level, array( '2', '4' ), true ) ? (int) $toc_level : 3;
        $is_toc_display = ( '0' !== $toc_level );
 
        preg_match_all( '/<h([1-6]).*?>(.+?)<\/h[1-6]>/s', $content, $headings, PREG_SET_ORDER );
        if ( empty( $headings ) ) {
            return $content;
        }
 
        $toc_html = custom_generate_toc_html( $headings, $target_level, $is_toc_display );
 
        if ( preg_match( '/<h2.*?>/i', $content, $matches ) ) {
            $content = preg_replace( '/<h2.*?>/i', $toc_html . $matches[0], $content, 1 );
        }
 
        return custom_add_heading_ids( $content, $headings, $target_level );
    }
    add_filter( 'the_content', 'custom_add_table_of_contents' );
}
 
if ( ! function_exists( 'custom_generate_toc_html' ) ) {
    function custom_generate_toc_html( $headings, $target_level, $is_toc_display ) {
        $toc_items = array();
 
        foreach ( $headings as $i => $heading ) {
            $level = (int) $heading[1];
            if ( $level > $target_level ) {
                continue;
            }
 
            $toc_items[] = sprintf(
                '<li><a href="#chapter-%d">%s</a></li>',
                $i,
                esc_html( $heading[2] )
            );
        }
 
        if ( empty( $toc_items ) ) {
            return '';
        }
 
        return sprintf(
            '<div class="table-of-contents" %s>
                <div class="toc-wrapper">
                    <div class="toc-title">目次</div>
                    <ul>%s</ul>
                </div>
            </div>',
            $is_toc_display ? '' : 'style="display:none;"',
            implode( "\n", $toc_items )
        );
    }
}
 
if ( ! function_exists( 'custom_add_heading_ids' ) ) {
    function custom_add_heading_ids( $content, $headings, $target_level ) {
        $i = 0;
        return preg_replace_callback(
            '/<h([1-6])(.*?)>(.+?)<\/h[1-6]>/s',
            function ( $matches ) use ( &$i, $target_level ) {
                $level = (int) $matches[1];
                if ( $level > $target_level ) {
                    return $matches[0];
                }
 
                return sprintf(
                    '<h%d id="chapter-%d"%s>%s</h%d>',
                    $level,
                    $i++,
                    $matches[2],
                    $matches[3],
                    $level
                );
            },
            $content
        );
    }
}

.table-of-contents {
  border: 1px solid #ddd;
  padding: 10px;
  margin-bottom: 20px;
  background: #f9f9f9;
}
.toc-wrapper {
  padding: 10px;
}
.toc-title {
  font-weight: bold;
  margin-bottom: 10px;
}