The documentation here is not fully correct.
<?php
    print_r(get_defined_constants());
   die;
?>
Will show basically:
  [PHP_YAF_VERSION] => 2.3.2
    [YAF_ENVIRON] => product
    [YAF_ERR_STARTUP_FAILED] => 512
    [YAF_ERR_ROUTE_FAILED] => 513
    [YAF_ERR_DISPATCH_FAILED] => 514
    [YAF_ERR_AUTOLOAD_FAILED] => 520
    [YAF_ERR_NOTFOUND_MODULE] => 515
    [YAF_ERR_NOTFOUND_CONTROLLER] => 516
    [YAF_ERR_NOTFOUND_ACTION] => 517
    [YAF_ERR_NOTFOUND_VIEW] => 518
    [YAF_ERR_CALL_FAILED] => 519
    [YAF_ERR_TYPE_ERROR] => 521
So use PHP_YAF_VERSION instead of just YAF_VERSION