WordPress/Gutenberg: Automatically create a list of links to anchor points in headings

functions.php

<?php

	//	Create an array to put out anchor links into

	$anchorLinks = array();

	//	We're going to parse out the blocks of the current page's content

	$blocks = parse_blocks(get_the_content());

	foreach ($blocks as $block) {
		//	We're only going through 'heading' blocks. Technically any(?) core Gutenberg block can have an anchor link, 
		//	but we're sticking with headings to keep it simple.
		
		if ($block['blockName'] == 'core/heading') {
			//	Only if the heading actually has an ID attr (which would indicicate it's an anchor)
			
			if (strpos($block['innerHTML'], 'id=') !== false) {

				//	User preg_match() to pull out the value of the ID, which in this case is $matches[1]
				
				preg_match('/id="([\s\S]*?)"/', $block['innerHTML'], $matches);

				//	Create an anchor link, with a link to the heading we've just processed, using the stripped text value 
				//	of the heading as the text for the link.
				
				$anchorLink = '<a href="#' . $matches[1] . '">' . wp_strip_all_tags($block['innerHTML']) . '</a>';

				//	Add it to our array
				
				$anchorLinks[] = $anchorLink;
			}
		}
	}

	//	Assuming the $anchorLinks array isn't empty, spit out an <ol> list of links

	if (!empty($anchorLinks)) {
		echo '<ol class="content-links sticky">';
		foreach($anchorLinks as $link)
			echo '<li>' . $link . '</li>';
		echo '</ol>';
	}
?>