post-thumbnail-fallback.php

<?php

/**
  Plugin Name: Post Thumbnail Fallback
  Plugin URI: https://github.com/BostonWP/functionality-plugins
  Description: Generates HTML for a posts thumbnail using multiple fallbacks to find an image.
  Version: .1
  Author: Jon Bishop
  Author URI: http://www.jonbishop.com
  License: GPLv2 or later
 */

function bwp_get_image_html($post_thumbnail_url, $size = 'thumbnail', $attr = '') {
    $src = $post_thumbnail_url;
    if (is_array($size)) {
        $width = $size[0];
        $height = $size[1];
    }
    $hwstring = image_hwstring($width, $height);
    if (is_array($size))
        $size = join('x', $size);
    $default_attr = array(
        'src' => $src,
        'class' => "attachment-$size"
    );

    $attr = wp_parse_args($attr, $default_attr);
    $attr = apply_filters('wp_get_attachment_image_attributes', $attr);
    $attr = array_map('esc_attr', $attr);
    $html = rtrim("<img $hwstring");
    foreach ($attr as $name => $value) {
        $html .= " $name=" . '"' . $value . '"';
    }
    $html .= ' />';
    return $html;
}


/**
 * Returns image HTML for a post with multiple fallbacks
 *
 * This function generates HTML for a posts thumbnail using multiple methods to
 * find an image.
 * 
 * First the function checks to see if there are any images attached the post.
 * If there is an image, we can use built core functions to generate the html.
 * 
 * Next the function checks to see if there are any images in the post's content.
 * If an image is found, the function resizes the image using the images.weserv.nl
 * web service.
 * 
 * Finally the function uses flickholdr.com to generate a relavent image using the
 * specifed dimensions.
 * 
 * Filters:
 * bwp_get_image_default - Define an alternative default image to flickholdr.com
 * bwp_get_image_html - Filter HTML generated by bwp_get_image()
 *
 * @param string $html Image HTML.
 * @param int $post_id The current post's ID
 * @param string|array $size Size of what the result image should be.
 * @return string Image HTML.
 */
function bwp_get_image($html = "", $post_id = null, $size = 'featured-image', $attr) {
    global $post, $_wp_additional_image_sizes;
    if ($html == "") {

        // First let's check for any attachments
        $attachments = get_children(array(
            'post_parent' => $post_id,
            'post_type' => 'attachment',
            'numberposts' => 1,
            'post_status' => 'inherit',
            'post_mime_type' => 'image',
            'order' => 'ASC',
            'orderby' => 'menu_order ASC'
                ));
        if (!empty($attachments)) {
            foreach ($attachments as $attachment_id => $attachment) {
                if (wp_get_attachment_image($attachment_id) != ""):
                    $post_thumbnail_url = wp_get_attachment_url($attachment_id);
                    $html = wp_get_attachment_image($attachment_id, $size, false, $attr);
                endif;
            }
        } else {
            // Create image dimensions for our custom image
            if (is_array($size)) {
                $width = $size[0];
                $height = $size[1];
            } else {
                if (isset($_wp_additional_image_sizes) && count($_wp_additional_image_sizes) && in_array($size, array_keys($_wp_additional_image_sizes))) {
                    $width = intval($_wp_additional_image_sizes[$size]['width']);
                    $height = intval($_wp_additional_image_sizes[$size]['height']);
                }
            }

            // Check for any images in the posts
            $output = preg_match_all('/<img.+src=[\'"]([^\'"]+)[\'"].*>/i', $post->post_content, $matches);
            if (!empty($matches[1][0])) {
                $post_thumbnail_url = $matches[1][0];
                $post_thumbnail_url = 'http://images.weserv.nl/?url=' . $post_thumbnail_url . '&h=' . $width . '&h=' . $height . '&t=fit';

                // Fall back image URL    
            } else {
                $f_tags = array();
                $query_tags = "";
                // Check for tags or categories to grab relevant images
                $categories = get_the_category();
                foreach ($categories as $category) {
                    $f_tags[] = $category->slug;
                    break;
                }
                $tags = get_the_tags();
                foreach ($tags as $tag) {
                    $f_tags[] = $tag->slug;
                    break;
                }
                $f_tags = array_diff($f_tags, array('uncategorized'));
                if(!empty($f_tags)){
                    $query_tags = implode(",", $f_tags);
                }
                $post_thumbnail_url = "http://flickholdr.com/" . $width . "/" . $height . "/" . $query_tags;
                $post_thumbnail_url = apply_filters('bwp_get_image_default', $post_thumbnail_url);
            }

            // Create image html with our custom image urls
            $html = apply_filters('bwp_get_image_html', bwp_get_image_html($post_thumbnail_url, array($width, $height), $attr));
        }
    }

    return $html;
}

function bwp_filter_post_thumbnail($html, $post_id, $post_thumbnail_id, $size, $attr) {
    return apply_filters('bwp_filter_post_thumbnail', bwp_get_image($html, $post_id, $size, $attr));
}

add_filter('post_thumbnail_html', 'bwp_filter_post_thumbnail', 10, 5);
?>