Import external url/image as WordPress attachment

gistfile1.php

<?php
	// Import the YouTube video thumbnail as the post's featured image
	// Only proceed if there is a youtube video thumbnail
	/*
		Example $youtube_video_data:
		array(
            'id'        => 'gvk6TwmF0cw',
            'title'     => 'Tyler Posey talks to Variety about being honored at Variety Power of Youth',
            'link'      => 'http://www.youtube.com/watch?v=gvk6TwmF0cw',
            'thumbnail' => 'http://i.ytimg.com/vi/gvk6TwmF0cw/hqdefault.jpg',
            'desc'      => 'Tyler Posey talks to Variety about being honored at Variety Power of Youth.',
            'duration'  => 33,
            'published' => '2013-07-30T16:52:41.000Z',
            'viewcount' => 1,
        )
	 */
	if ( isset( $youtube_video_data['thumbnail'] ) ) {
		if ( ! empty( $youtube_video_data['thumbnail'] ) ) {
			
			// Does the post already have a featured image? 
			if ( has_post_thumbnail( $post->ID ) ) {
				// Yes, it does have a featured image, DO NOT bring the image from YouTube over
			} else {
				// NO, it does not have a featured image, let's import the image from YouTube
				
				// Fetch the upload directory location
				$upload_directory = wp_upload_dir();

				// Acertain the image filename
				$upload_filename = basename( $youtube_video_data['thumbnail'] );

				// Build the upload path
				$upload_dir_w_filename = $upload_directory['path'] . $upload_filename;

				// Fetch the remote file
				$remote_file = file_get_contents( $youtube_video_data['thumbnail'] );

				// Open a local file stream for writing
				$save_remote_file_locally = fopen( $upload_dir_w_filename, 'w' );

				// Write out the remote file to our local stream
				fwrite( $save_remote_file_locally, $remote_file );

				// And close the stream
				fclose( $save_remote_file_locally );

				// Let's see what we've fetched..
				$wp_filetype = wp_check_filetype( $upload_dir_w_filename );

				// Let's build the new attachment arguments for insertion into the media library
				$attachment_id = wp_insert_attachment( array(
						'post_mime_type' => $wp_filetype['type'],
						'post_title'     => $youtube_video_data['title'],
						'post_content'   => '',
						'post_status'    => 'inherit',
					), 
					$upload_dir_w_filename, $post->ID );

				// Fetch the WordPress attachment file
				$attachment_file = get_attached_file( $attachment_id );

				// Ensure the WP image library is loaded
				require_once ( ABSPATH . 'wp-admin/includes/image.php' );

				// Build the attachment meta data
				$attachment_meta_data = wp_generate_attachment_metadata( $attachment_id, $attachment_file );

				// Bind the new meta data to the attachment
				wp_update_attachment_metadata( $attachment_id, $attachment_meta_data );

			} // if has featured image
		} // if yt image not empty
	} // if yt image isset