Counting substring occurrences in PostgreSQL

I got to count occurrences of / character today and found out no built-in function exists in PostgreSQL, so here’s my shot at it. Pretty simple, yet useful.

CREATE OR REPLACE FUNCTION how_many(IN text, IN varchar, OUT integer)
RETURNS integer
AS
$how_many$
    SELECT length($1) - length(replace($1, $2, ''));
$how_many$
LANGUAGE SQL
SECURITY DEFINER;

-- SELECT how_many('test', 't'); -- returns number 2