Initial coalesce operator implementation
Added test
Ensure not evaluated twice
Extended coalesce operator test case for ordering/short-circuiting
Merge branch 'coalesce_operator'

* coalesce_operator:
  Extended coalesce operator test case for ordering/short-circuiting
  Ensure not evaluated twice
  Added test
  Initial coalesce operator implementation
Merge branch 'master' of git://github.com/php/php-src