You can use the date function:
date("Y-m-d H:i:s");
date('Y-m-d H:i:s')
Look here for more details: http://pl.php.net/manual/en/function.date.php
With PHP version >= 5.4 DateTime can do this:-
echo (new \DateTime())->format('Y-m-d H:i:s');
See it working.