How to declare a variable in a PostgreSQL query

How do I declare a variable for use in a PostgreSQL 8.3 query?

In MS SQL Server I can do this:

DECLARE @myvar INT
SET @myvar = 5

SELECT *
FROM somewhere
WHERE something = @myvar

How do I do the same in PostgreSQL? According to the documentation variables are declared simply as “name type;”, but this gives me a syntax error:

myvar INTEGER;

Could someone give me an example of the correct syntax?