<?php
$string = "2019-08-10D00:00:03.712125000";
$parts = explode(".", $string);
$microseconds = $parts[1];
$date = DateTime::createFromFormat("Y-m-d\DH:i:s", $parts[0]);
$timestamp = $date->getTimestamp();
// If you want to round it to 4 decimal places
// $microseconds = ltrim(round("0.".$microseconds, 4), "0.");
// If you want to append the microseconds back for whichever reason
// $timestamp .= ".".$microseconds;
echo $timestamp;