This works even if you are on 32-bit PHP:
list($msec, $sec) = explode(' ', microtime());
$time_milli = $sec.substr($msec, 2, 3); // '1491536422147'
$time_micro = $sec.substr($msec, 2, 6); // '1491536422147300'
Note this doesn't give you integers, but strings. However this works fine in many cases, for example when building URLs for REST requests.
If you need integers, 64-bit PHP is mandatory.
Then you can reuse the above code and cast to (int):
list($msec, $sec) = explode(' ', microtime());
// these parentheses are mandatory otherwise the precedence is wrong!
// ? ?
$time_milli = (int) ($sec.substr($msec, 2, 3)); // 1491536422147
$time_micro = (int) ($sec.substr($msec, 2, 6)); // 1491536422147300
Or you can use the good ol' one-liners:
$time_milli = (int) round(microtime(true) * 1000); // 1491536422147
$time_micro = (int) round(microtime(true) * 1000000); // 1491536422147300