Generating 16 digits timestamp for nonce in php

How to generate 16 digits timestamp in php, i want to use it for nonce(unique number that you send everytime you request from server to prevent hack) for api.

I tried using time() function but it returns only 10 digits of number. here is the example

time();//returns 1532708348

My question is how to generate the result into 1532708348690900? and whats the meaning of last 6 digits?

Thanks in advance :D