Optimiser logic addition. Functions elimination

[NickSettler]

Code optimiser must be able to remove unused functions declaration. Function is unused when it is not used anywhere in the code except its own scope. For example, the fib function can be removed from the example below. It is used in its own block of code, but it is not used anywhere else in the code, so it is unused.

function fib(int $x): int {
  if (x == 1 || x == 0) {
      return(x);
   } else {
      return fib(x - 1) + fib(x - 2);
   }
}

$a = 2;
write($a);