PostgreSQL equivalent for MySQL variable definition within a query

Question

I am looking for a way how to dynamically define and assign a value/-s for a variable in PostgreSQL within a query.

A very primitive MySQL example which should be translated into PostgreSQL would be as follows:

SELECT
  myVar1 := 15,
  myVar2 := 10,
  @myVar1 + @myVar3 AS myVar1_plus_myVar2

Note that everything is declared within SELECT section.