During my thirty years as a mainframe programmer, twenty years included writing SQL queries under DB2. DB2 signs a contract whereby the "current_timestamp" will never return two timestamps with the same date/time.
This is the very reason why a timestamp is the perfect primary index for a log table, not only does it provide the when, and ensure you can reference the log entries in sequence, but it is UNIQUE.
I based an ETL on incoming entries being processed in arrival timestamp sequence because of this handshake. For this to happen, we need 6-digit millisecond precision in the timestamp.
The PHP date("Y-m-d H:i:s") function does not seem to provide more than 2-digit "seconds" precision.
WHY!? Someone like me would say... why not just attach attach another some identifier to the end of the mask, and get back the milliseconds.
This is Unix timestamp; "2018-05-25 12:35:29:123456", in fact the space between the date and time looks odd to me as well; as does the colon between the seconds and milliseconds. This is what is ideal in my mind... "2015-05-25-184.108.40.206456" a 26-byte value that represents the current date/time, and will never be a duplicate of another timestamp; no spaces, no colons, no inconsistent special characters.
Of course that is what you will get when standards are not cobbled together by an incoherent effort.
I have seen this question asked in a number of places. How do I get a current timestamp from php "WITH" milliseconds, and the response from the internet geniuses is one to a completely different question... How do I get a current timestamp "IN" milliseconds. NOT THE SAME QUESTION!!! is it?
So does anyone have an answer that doesn't require some ridiculous concoction of code in a cryptic function that no one will ever make sense of? Or, in your unbounded need to post a response will you post a response even if you didn't read the question in question?
I'm sorry, I spend way too much time seeking answers and never finding one. Forgive my rant.