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;
} );