Retrieve 1st image in post and set it as featured image, when post is saved/updated

Question

This might work for you as well. It scans the post, then pulls in the image set as ‘featured’. If that doesn’t exist, it returns the first image attached to the post. If there isn’t one, it looks for any image in the post; if that’s not there, it grabs a YouTube screenshot. Put this in your theme’s functions.php file:

// Note that your theme must support post thumbnails for this function to work. 
// If you are getting an error try adding add_theme_support('post-thumbnails'); to your functions. php file  
function vp_get_thumb_url($text, $size){
    global $post;
    $imageurl="";

    // Check to see which image is set as "Featured Image"
    $featuredimg = get_post_thumbnail_id($post->ID);
    // Get source for featured image
    $img_src = wp_get_attachment_image_src($featuredimg, $size);
    // Set $imageurl to Featured Image
    $imageurl=$img_src[0];

    // If there is no "Featured Image" set, move on and get the first image attached to the post
    if (!$imageurl) {
        // Extract the thumbnail from the first attached imaged
        $allimages =&get_children('post_type=attachment&post_mime_type=image&post_parent=" . $post->ID );

        foreach ($allimages as $img){
            $img_src = wp_get_attachment_image_src($img->ID, $size);
            break;
        }
        // Set $imageurl to first attached image
        $imageurl=$img_src[0];
    }

    // If there is no image attached to the post, look for anything that looks like an image and get that
    if (!$imageurl) {
        preg_match("/<\s*img [^\>]*src\s*=\s*[\""\']?([^\""\'>]*)/i' ,  $text, $matches);
        $imageurl=$matches[1];
    }

    // If there's no image attached or inserted in the post, look for a YouTube video
    if (!$imageurl){
        // look for traditional youtube.com url from address bar
        preg_match("/([a-zA-Z0-9\-\_]+\.|)youtube\.com\/watch(\?v\=|\/v\/)([a-zA-Z0-9\-\_]{11})([^<\s]*)/", $text, $matches2);
        $youtubeurl = $matches2[0];
        $videokey = $matches2[3];
    if (!$youtubeurl) {
        // look for youtu.be 'embed' url
        preg_match("/([a-zA-Z0-9\-\_]+\.|)youtu\.be\/([a-zA-Z0-9\-\_]{11})([^<\s]*)/", $text, $matches2);
        $youtubeurl = $matches2[0];
        $videokey = $matches2[2];
    }
    if ($youtubeurl)
        // Get the thumbnail YouTube automatically generates
        // '0' is the biggest version, use 1 2 or 3 for smaller versions
        $imageurl = "http://i.ytimg.com/vi/{$videokey}/0.jpg";
    }

    // Spit out the image path
    return $imageurl;
}

To display this in your theme files, first insert the following:

<?php
if (function_exists('vp_get_thumb_url')) {
                                // Set the desired image size. Swap out 'thumbnail' for 'medium', 'large', or custom size
                                $thumb=vp_get_thumb_url($post->post_content, 'large-feature'); 
                            }
?>

and somewhere after that, to display the actual image, in this case wrapped with a link to the post:

<?php if ($thumb!='') { ?>
                                        <a href="https://wordpress.stackexchange.com/questions/45529/<?php the_permalink() ?>" title="<?php the_title(); ?>" rel="bookmark"><img src="<?php echo $thumb; ?>" alt="<?php get_the_title(); ?>" /></a>
                                    <?php } ?>

Search Google on the vp_get_thumb_url function name for other articles on how to implement this. Best of luck!

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