Basic SQL Function question + existance of SPL?

Apologies if this question is too simple, but I couldn't find an example
in the manual.

If I'm writing a function in SQL, how do I use the value of a passed
argument? For example, consider:

CREATE FUNCTION foo(float8) RETURNS float8 AS
'select 1.0 as result' language 'sql';

select foo(2.3) as answer;

This works great, but I'd like to write a function where the result
actually depends on the argument-- as a simple example, I'd like to get
back the argument plus 1 (eg, "select foo(2.3) as answer" yields 3.3 where
as "select foo(4.3) as answer" yields 5.3, and so on. How do I do this?

Also, does PostgreSQL support SPL or any sort of procedural language? I'd
like to write the following (highly-recursive) function:

f(x) = (x*x/120-1/6)*x*x+1)*x for 0<=x<1.57
f(x) = f(3.14-x) for 1.57<=x<=3.14
f(x) = -f(x-3.14) for 3.14<=x<6.28
f(x) = f(x-6.28) for x>=6.28
f(x) = f(x+6.28) for x<0

(this is roughly the sin() function).

In SPL, I could write this something like:

IF (x<1.57) RETURN (SELECT (x*x/120-1/6)*x*x+1)*x as ANSWER);
IF (x>=1.57 AND x<=3.14) RETURN (SELECT f(3.14-x) as ANSWER);
...

and so on. Does PostgreSQL support anything like that? If not, is there an
SQL query which will give me the function I want?