function expire( $key ) {
            $this->_logger->entrance( 'key = ' . $key );

            $cache_entry = $this->_options->get_option( $key, false );

            if ( is_object( $cache_entry ) &&
                 isset( $cache_entry->timestamp ) &&
                 is_numeric( $cache_entry->timestamp )
            ) {
                // Set to expired.
                $cache_entry->timestamp = WP_FS__SCRIPT_START_TIME;
                $this->_options->set_option( $key, $cache_entry, true );
            }
        }