Time of the current request in seconds elapsed since the Unix Epoch.
This differs from $_SERVER['REQUEST_TIME'], which is stored as a float since PHP 5.4.0. Float timestamps confuse most PHP functions (including date_create()).
http://php.net/manual/reserved.variables.server.php
http://php.net/manual/function.time.php
define('REQUEST_TIME', (int) $_SERVER['REQUEST_TIME']);