There is no native function for a unix timestamp in milliseconds, only 'time()' which returns seconds since epoch and 'microtime()', which returns microseconds since the current second. Although you can use 'microtime(true)', but it returns a float that can not guarantee precision. Or just 'microtime()' and explode the string.
So I was for once in a long time again busy with web development, trying out the new PHP 7 in combination with xampp. Being frustrated by the slow page servings on my decent hardware equipped localhost, I went out to find the bottleneck.