Help for plpgsql!!

Dear Sir/Madam,
My table is :
Attribute | Type | Modifier
-----------+-----------+----------
empname | text |
salary | integer |
last_date | timestamp |
last_name | name |
My function and trigger is:
create function isFull()
returns opaque as '
begin
select count(*) into tmp1 from emp;
if tmp1.count >= 5 then
select min(last_date) into tmp2 from emp;
delete from emp where last_date=tmp2.min;
drop table tmp2;
end if;
drop table tmp1;
end; '
language 'plpgsql';
create trigger isfull before insert on emp for each row execute procedure
isFull
();

But my error messages is :
ERROR: parser: parse error at or near "tmp1"

I can finish these steps in psql environment,why it could not work well in
plpgsql?Can you help me?
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