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)
intotmp2 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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