udf return column name not value

[chamila gayan]

CREATE FUNCTION getcolumnvalue(id int,columnname varchar(30))
RETURNS varchar(50) DETERMINISTIC
READS SQL DATA
begin
declare retval varchar(50);
select columnname into retval from user where ID = id ;
return retval;
end;

I want get value of related column but it return column name.
ex:- ('tom' what I want but it return 'name')
plz tell what the wrong of this

thank you