Gmdate в PHP

<?php
// PHP program to illustrate gmdate function
// display date Jun 25 2018 23:21:50
echo gmdate("M d Y H:i:s", mktime(23, 21, 50, 6, 25, 2018)) ."\n";

// display date  World Wide Web Consortium
// 2018-06-25T23:21:50+00:00
echo gmdate(DATE_W3C, mktime(23, 21, 50, 6, 25, 2018)). "\n";

// display date as ISO-8601 format
echo gmdate(DATE_ISO8601, mktime(23, 21, 50, 6, 25, 2018)). "\n";
?>
Undefined