Converting human-friendly date to milliseconds

strtotime($human_readable_date) * 1000

Pay attention: strtotime() * 1000 is ok to have seconds expressed as milliseconds!

The right answer is that it is not possible to have a millisecond precision on date/time functions in PHP. The precision of Unix Epoc based functions is only of 1k milliseconds, aka second :)

Using the suggested answers you don't have milliseconds, but seconds expressed as number of milliseconds.

If you are aware of this, and you don't really need a millisecond precision then the answers given are ok, but the question was wrong :)