Is this even possible in PHP?
If not, what is the highest precision available?
The microtime function is what you're looking for.
PHP does not supply a function that has higher precision than microseconds.
You can use the system function to get the value straight from the machine if you are running Linux:
$nanotime = system('date +%s%N');
%s is the amount of seconds, appended by %N, which is the amount of nanoseconds.
microtime() is the highest precision using PHP's vocabulary. You can also make a system call by using Unix date and specify %N format if you need nanosecond accuracy. Unix ntp_gettime() has even higher resolution and returns jitter/wander/stability/shift/calibration data.
Now with this extension it's nanosecond timing is possible http://pecl.php.net/hrtime . Yet it is a stopwatch class implemented to suite the highest possible resolution on different platforms.
As of PHP 7.3 there's also a crossplatform hrtime() function in the core http://php.net/manual/en/function.hrtime.php.
来源：https://stackoverflow.com/questions/4207343/how-to-get-time-in-php-with-nanosecond-precision