Simple Table Of Contents

add_filter( 'the_content', function ( $content ) {

	// This snippet requires the DOMDocument class to be available.

	if ( ! class_exists( 'DOMDocument' ) ) {

		return $content;

	}

	if ( !is_single() || !in_the_loop() || !is_main_query() ) {

		return $content;

	}

	$dom          = new DOMDocument();

	$load_content = mb_convert_encoding( $content, 'HTML-ENTITIES', 'UTF-8' );

	if ( empty( $load_content ) ) {

		return $content;

	}

	@$dom->loadHTML( $load_content );

	$xpath    = new DOMXPath( $dom );

	$headings = $xpath->query( '//h2 | //h3' );

	// Let's loop through all the headings and add them to the table of contents.

	$headings_list = '<ul class="table-of-contents">';

	foreach ( $headings as $heading ) {

		$heading_id = $heading->getAttribute( 'id' );

		if ( empty( $heading_id ) ) {

			// Generate a heading id and add it to the heading.

			$old_heading = $heading->C14N();

			$heading_id  = sanitize_title( $heading->nodeValue );

			$heading->setAttribute( 'id', $heading_id );

			$content = str_replace( $old_heading, $heading->C14N(), $content );

		}

		$heading_text  = $heading->nodeValue;

		$headings_list .= '<li><a href="#' . $heading_id . '">' . $heading_text . '</a></li>';

	}

	$headings_list .= '</ul>';

	$content       = $headings_list . $content;

	return $content;

} );