WP: Ultimate fallback solution to getting a thumbnail for a post

getPostThumbnail.php

<?php

function nb_get_post_thumbnail( $post_id = null, $size = 'medium', $attr = [] ) {
    if(!$post_id) {
        $post_id = get_the_ID();
    }

    // Step 1: Use the featured image if it's there
    if(!empty($image = get_the_post_thumbnail($post_id, $size, $attr))) {
        return $image;
    }

    // Step 2: Try to get the first cover image
    $content = get_post_field('post_content', $post_id);

    // Get the the first cover, image, or media block you can find
    preg_match('/<!-- (wp:cover|wp:image|wp:media-text) (.*)\/(?:\1) -->/ms', $content, $imageBlockMatches);
    if(isset($imageBlockMatches[ 2 ])) {
        preg_match('/(?:"id"|"mediaId")(?:\D*)(\d*)/', $imageBlockMatches[ 2 ], $imageId);
        if(!empty($image = wp_get_attachment_image($imageId[ 1 ], $size, false, $attr))) {
            return $image;
        }
    }

    // The easy part is over. Now we need to delve into the rendered content!
    $content = apply_filters('the_content', $content);
    $content = str_replace(']]>', ']]&gt;', $content);

    // Get the first img tag's src attribute you see, then remove the -200x200 part, get the ID from the URL, Whabam
    preg_match('/(?:<img)(?:.*?)(?:src=["|\'])(.*?)["|\']/ms', $content, $srcMatch);
    if(isset($srcMatch[ 1 ]) && !empty($srcMatch[ 1 ])) {
        $baseUrl = preg_replace('/(.*)(-\d*x\d*)/', '$1', $srcMatch[ 1 ]);
        if(!empty($imageId = attachment_url_to_postid($baseUrl))) {
            if(!empty($image = wp_get_attachment_image($imageId, $size, false, $attr))) {
                return $image;
            }
        }
    }

    // If there is legitimately no image on your page, get the logo of the site as a final fallback.
    if(empty($image)) {
        if(!empty($image = wp_get_attachment_image(get_theme_mod('custom_logo'), $size, false, $attr))) {
            return $image;
        }
    }

    // And if there really is nothing...sorry =(
    return '';
}