$req['nonce'] = substr($mt[1],-4).substr($mt[0], 2, 6);
Just make sure that they don't require ever-increasing nonce (but they probably do), because your method will create numbers starting back from 0 after couple of hours. What you have now is a number which increases for 10000 seconds but then starts back from 0. You also don't ensure that you won't generate a nonce that has already been used in the past, as it's possible you'll get a colliding number one day. This is a very dirty hack what you have now.