Output thumbnail address

//输出缩略图地址
function post_thumbnail_src() {
    global $post;
    if ($values = get_post_custom_values("thumb")) {   //输出自定义域图片地址
        $values = get_post_custom_values("thumb");
        $post_thumbnail_src = $values [0];
    } elseif (has_post_thumbnail()) {    //如果有特色缩略图，则输出缩略图地址
        $thumbnail_src = wp_get_attachment_image_src(get_post_thumbnail_id($post->ID), 'full');
        $post_thumbnail_src = $thumbnail_src [0];
    } else {
        $post_thumbnail_src = '';
    };
    echo $post_thumbnail_src;
}