Noptin_Page::email_open( $filter )

	public function email_open( $filter ) {

		if ( 'email_open' != $this->get_request_action() ) {
			return $filter;
		}

		if ( isset( $_GET['sid'] ) && isset( $_GET['cid'] ) ) {
			$subscriber_id = intval( $_GET['sid'] );
			$campaign_id   = intval( $_GET['cid'] );
			log_noptin_subscriber_campaign_open( $subscriber_id, $campaign_id );
		}

		// Display 1x1 pixel transparent gif.
		nocache_headers();
		header( 'Content-type: image/gif' );
		header( 'Content-Length: 42' );
		echo base64_decode( 'R0lGODlhAQABAID/AMDAwAAAACH5BAEAAAAALAAAAAABAAEAAAICRAEA' );
		exit;

	}