A plugin for Wordpress and NexGen Gallery that use a metabox to select a NextGEN Gallery and use this gallery Preview Image as Post Featured Image

GMNextGenPreviewImage

<?php
/**
* @package GMNextGenPreviewImage
* @version 0.1.0
*/
/**
Plugin Name: GM NextGen Preview Image
*
Author: Giuseppe Mazzapica
Version: 0.1.0
Description: Use a metabox to select a NextGEN Gallery and use this gallery Preview Image as Post Featured Image
*/



/**
* GMNextGenPreviewImage class
*
* @package GMNextGenPreviewImage
* @author Giuseppe Mazzapica
*
*/
class GMNextGenPreviewImage {


/**
* Plugin version
*
* @since	0.1.0
*
* @var	string
*/
protected $version = '0.1.0';


/**
* A prefix
*
* @since	0.1.0
*
* @var	string
*/
protected static $meta = '_GMNpreview';



/**
* Retrieve the thumbnail from meta field and save option
*
* @since	0.1.0
*
* @access protected
* @return null
*/
protected static function get_thumb($post_id = 0,  $gallery_id = '') {
global $wpdb;
$galleries = $wpdb->prefix . 'ngg_gallery';
$images = $wpdb->prefix . 'ngg_pictures';
$gallery = $wpdb->get_row( $wpdb->prepare("SELECT * FROM $galleries WHERE gid = %d", $gallery_id) );
if ( $gallery ) {
if ( intval($gallery->previewpic) ) {
$thumb = $wpdb->get_row( $wpdb->prepare("SELECT * FROM $images WHERE pid = %d", $gallery->previewpic) );
} else {
$thumb = $wpdb->get_row( $wpdb->prepare("SELECT * FROM $images WHERE galleryid = %d ORDER BY sortorder ASC LIMIT 1", $gallery_id) );
}

if ( $thumb ) {
$url = site_url( trailingslashit($gallery->path) . $thumb->filename );
$title = $thumb->alttext;
self::save_thumb($post_id, $url, $title);
}
}
}


/**
* Upload the image and set it as thumbnail
*
* @since	0.1.0
*
* @access protected
* @return null
*/
protected static function save_thumb ($post_id = 0, $url = '', $title = '') {
if ( empty($post_id) || empty($url) || ! filter_var($url, FILTER_VALIDATE_URL) ) return;
$saved = (array)get_option('GMNextGenPreviewImage');
if ( $saved && in_array($url, array_values($saved) ) ) {
$thumbnail_id = array_search ($url , $saved, true);
} else {
// download and save thumb
$get = wp_remote_get( $url );
$mime_type = wp_remote_retrieve_header( $get, 'content-type' );

if ( ! substr_count($mime_type, 'image') ) return false;
$bits = wp_upload_bits( basename($url), '', wp_remote_retrieve_body( $get ) );

if ( ! empty($bits['error']) ) return false;
$thumb_data = array(
'post_title'=> $title ? : __('Thumbnail'), 'post_mime_type' => $mime_type, 'post_content' => ''
);   
$thumbnail_id  = wp_insert_attachment( $thumb_data, $bits['file'], $post_id ); 
if ( $thumbnail_id ) {
$saved[$thumbnail_id] = $url;
update_option('GMNextGenPreviewImage', $saved);
require_once(ABSPATH . 'wp-admin/includes/image.php');
$metadata = wp_generate_attachment_metadata( $thumbnail_id, $bits['file'] );
wp_update_attachment_metadata( $thumbnail_id, $metadata );

}
}
if ($thumbnail_id) set_post_thumbnail( $post_id, $thumbnail_id );
}



/**
* Register the metabox
*
* @since	0.1.0
*
* @access public
* @return null
*/
static function metabox() {
global $post;
$types = (array)apply_filters('gmngpi_meta_key', array('post') );
if ( ! has_post_thumbnail($post->ID) && in_array($post->post_type, $types) ) {
add_meta_box('gmngpi_meta_box', __('Choose Gallery'), array(__CLASS__, '_metabox'), $post->post_type );
}
}



/**
* Print the metabox html
*
* @since	0.1.0
*
* @access public
* @return null
*/
static function _metabox( $post ) {
if ( ! has_post_thumbnail($post->ID) ) {
wp_nonce_field( plugin_basename( __FILE__ ), self::$meta . '_nonce' );
global $wpdb;
$galleries = $wpdb->get_results( "SELECT gid, name FROM " . $wpdb->prefix . 'ngg_gallery' );
if ( ! empty( $galleries) ) {
echo '<select id="' . self::$meta . '_meta_id" name="' . self::$meta . '">';
echo '<option value="" selected="selected">---</option>';
foreach ( $galleries as $gallery ) {
printf('<option value="%d">%s</option>', $gallery->gid, $gallery->name);
}
echo '</select>';
}
} else {
_e('The post already has a Featured Image');
}
}



/**
* Save the metadata
*
* @since	0.1.0
*
* @access public
* @return null
*/
static function set_thumb( $post_id ) {
if ( ! has_post_thumbnail($post_id ) ) {
if ( ! isset( $_POST[ self::$meta . '_nonce' ] ) || ! wp_verify_nonce( $_POST[ self::$meta . '_nonce' ], plugin_basename( __FILE__ ) ) ) return;
$types = (array)apply_filters('gmngpi_meta_key', array('post') );
if ( ! in_array($_REQUEST['post_type'], $types) || ! current_user_can( 'edit_posts', $post_id ) ) return;
$posted = is_numeric($_POST[self::$meta]) ? intval($_POST[self::$meta]) : null;
if ( ! empty($posted) ) self::get_thumb($post_id, $posted);
}
}


/**
* Register hook to inizializate plugin on loop start
*
* @since	0.1.0
*
* @access public
* @return null
*/	
static function init() {
add_action( 'add_meta_boxes', array(__CLASS__, 'metabox') );
add_action( 'save_post', array(__CLASS__, 'set_thumb') );
}



/**
* Remove option
*
* @since	0.1.0
*
* @access public
* @return null
*/	
static function uninstall() {
delete_option('GMNextGenPreviewImage');
}



/**
* Constructor. do nothing.
*
* @since	0.1.0
*
* @access	public
* @return	null
*/
function __construct( ) {

}
}



/**
* Init function. Check for NextGen 2.x and if there inizialize plugin.
*
* @since	0.1.0
*
* @access	public
* @return	null
*/
function initGMNextGenPreviewImage() {
if( class_exists('C_NextGEN_Bootstrap') ) {
GMNextGenPreviewImage::init();
}
}



/**
* Once plugins_loaded is the earliest hook, use lower priority to easy remove
*
*/
add_action('plugins_loaded', 'initGMNextGenPreviewImage', 100);


/**
* Unistall And deactivation hooks
*
*/
register_deactivation_hook(__FILE__, array('GMNextGenPreviewImage', 'uninstall'));
register_uninstall_hook(__FILE__, array('GMNextGenPreviewImage', 'uninstall'));