Pl/pgsql function

I am implementing some pl/pgsql functions.

Is there any way to change the input
for example- I got some value by $1. I want to modify this value (means split that value), Can we do this and how?

Second thing,
Suppose i defined a function test as

select test('geom',the_geom,time) from tablename
.....
Inside body,
How can i get the tablename inside the body (because i haven't pass table name to function)
......
end