Output thumbnail address

//Output thumbnail address

function post_thumbnail_src() {

global $post;

if ($values ​​= get_post_custom_values("thumb")) { //Output custom domain image address

$values = get_post_custom_values("thumb");

$post_thumbnail_src = $values ​​[0];

} elseif (has_post_thumbnail()) { //If there is a featured thumbnail, output the thumbnail address

$thumbnail_src = wp _get_attachment_image_src(get_post_thumbnail_id ($post->ID), 'full');

             $post_thumbnail_src = $thumbnail_src [0];

                                                                                                                                                           ($post->ID), 'full');

echo $post_thumbnail_src ;

}