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
