DROP PROCEDURE `GET_CATEGORY_ID_BY_NAME`;
CREATE DEFINER=`root`@`localhost` PROCEDURE `GET_CATEGORY_ID_BY_NAME`(IN `category_name` VARCHAR(255) CHARSET utf8, OUT `categoryID` INT) NOT DETERMINISTIC MODIFIES SQL DATA SQL SECURITY DEFINER BEGIN
	
    SELECT category_id INTO categoryID FROM `category_description` WHERE  `name`=category_name;
	
    IF ISNULL(categoryID) THEN 
    	INSERT INTO `category` (parent_id, sort_order, status) VALUES (0,1,1);
        SET categoryID := LAST_INSERT_ID();
        
	INSERT INTO `category_description` (category_id, language_id, `name`) VALUES (categoryID,1,category_name);
    	INSERT INTO `category_to_store` (category_id, store_id) VALUES (categoryID,1,0);
    END IF;
END

With an argument, LAST_INSERT_ID() returns an unsigned integer.