Dynamic SQL is not allowed in stored function or trigger

Question

I am trying to create a dynamic function using mysql, And getting error. How can i solve it.

CREATE function updateTable(user_name Varchar(50), item_name Varchar(50), item_val Varchar(50)) RETURNS int
    BEGIN
    SET @item_val = item_val;
    SET @item_name = item_name;
    SET @user_name = user_name;
    IF @item_val = '' THEN
    SET @myCommand = concat('UPDATE table SET @item_name = null , row_modified =  CURRENT_TIMESTAMP WHERE user_id = @user_name;');
    PREPARE stmt FROM @myCommand;
    EXECUTE stmt;
    DEALLOCATE PREPARE stmt;
    ELSE
    SET @myCommand = concat('UPDATE table SET @item_name = @item_val , row_modified =  CURRENT_TIMESTAMP WHERE user_id = @user_name;');
    PREPARE stmt1 FROM @myCommand;
    EXECUTE stmt1;
    DEALLOCATE PREPARE stmt1;
    END IF;
    RETURN 1;

    END;