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] => 521So use PHP_YAF_VERSION instead of just YAF_VERSION