Description:
------------
Currently PHP supports global constant definition using keyword const instead of ugly define() function call.

The problem is that const definition doesn't allow expressions to be used to define a constant.

So please implement the ability to use expressions when using const keyword.

Test script:
---------------
<?php
  const TIMEOUT = 10 * 60;

  const SITE_ROOT = __DIR__;
  const PAGES_DIR = '/pages/';
  const PAGES_ROOT = SITE_ROOT . PAGES_DIR;

Expected result:
----------------
All constants defined.

Actual result:
--------------
Parse error:  syntax error, unexpected '*', expecting ',' or ';' in ...