Noptin_Mailer::get_tracker( $data = array() )

	public function get_tracker( $data = array() ) {

		if ( empty( $data['campaign_id'] ) || empty( $data['subscriber_id'] ) ) {
			return '';
		}

		$url = get_noptin_action_url( 'email_open' );

		$url = add_query_arg(
			array(
				'sid' => intval( $data['subscriber_id'] ),
				'cid' => intval( $data['campaign_id'] ),
			),
			$url
		);

		$url = esc_url( $url );

		return "<img src='$url' style='border:0;width:1px;height:1px;' />";

	}