define_syslog_variables() only defines global variables. Constants are already always defined, if the syslog module is loaded. You _do not_ need to call this to use the syslog constants.
For instance, on my system:
<?php
var_dump(LOG_ERR); // int(3)
var_dump($LOG_ERR); // NULL (and an E_NOTICE)
define_syslog_variables();
var_dump($LOG_ERR); // int(3)
?>
debugger_on