Get site links in WordPress post content to use as related posts.

wp_make_related_posts_from_content_links.php

<?php
$query = new WP_Query( array( 'post_type' => 'post', 'posts_per_page' => 500 ) );

/**
 * Update post meta with IDs of URLs in post content.
 *
 * Uses post content links as related post.
 */
foreach ( $query->posts as $post ) {
	$post_id = $post->ID;
	$content = $post->post_content;
	$url_ids = array();
  // Get site URLs in post content.
	$urls   = my_get_post_urls_in_content( $content );
			
	foreach ( $urls as $url ) {
    // Attempt to get post ID from URL.
		$url_id = url_to_postid( $url );
    // If ID is non-zero, add to array.
		if ( $url_id ) {
			$url_ids[] = $url_id;
		}
		// print_r( $url_ids );
		// If using ACF field.
		// $update_field = update_field( 'related_posts', $url_ids, $post_id );
    // If native CF.
    // $update_meta = update_post_meta( $post_id, 'related_posts', $url_ids );
	}	
}

/**
 * Extract interal (site) URLs from string (e.g., post content).
 *
 * @param string $content A string with URLs to extract.
 * @return array $urls An array of site URLs.
 */
function my_get_post_urls_in_content( $content ) {
	$site_url  = site_url();
	// Make absolute URLs from relative URLs.
	$content   = str_replace( 'href="/', "href=\"$site_url/", $content );
	// Get arrray of URLs from content.
	$urls_all  = wp_extract_urls( $content );
	// Keep only URLs from this site.
	$urls = array_filter( $urls_all, function ( $var ) use ( $site_url ) {
		return ( strpos( $var, $site_url ) !== false );
	});
	
	return $urls;
}
?>