public function get_pum_db_ver() {

    	static $pum_db_ver;

    	if ( ! isset( $pum_db_ver ) ) {
		    // this is the current database schema version number
		    $pum_db_ver = pum_get_db_ver();
	    }

        if ( ! $pum_db_ver ) {
            $this->set_pum_db_ver();
            $pum_db_ver = pum_get_db_ver();
        }

        return preg_replace( '/[^0-9.].*/', '', $pum_db_ver );
    }