Measuring a duration with microtime results randomly in zero

I am having a loop like this:

<?php
ini_set('memory_limit', '16024M');
ini_set('set_time_limit', 9999);
ini_set('max_execution_time', 9999);
ini_set('display_errors',  TRUE);
ini_set('error_reporting',  E_ALL);

for ($k = 1; $k <= 50; $k++) {

    $haystack = array();

    for ($i = 1; $i <= 100; $i++) {

        $randomChar = substr(md5(microtime()),rand(0,26), 1);

        $haystack[] = $randomChar;

    }

    $haystack[] = 'X';

    $startTime = microtime(true);

    // sleep(0);

    $result = in_array('X', $haystack);

    $endTime = microtime(true);

    echo number_format(1000000 * ($endTime - $startTime), 20, ",", " ") . ' ';

 }

And these are the first couple of lines from the output:

1,90734863281250000000 0,95367431640625000000 1,19209289550781250000 1,90734863281250000000 1,19209289550781250000 0,95367431640625000000 0,95367431640625000000 1,90734863281250000000 0,95367431640625000000 20,02716064453125000000 0,95367431640625000000 1,19209289550781250000 0,95367431640625000000 0,95367431640625000000 0,00000000000000000000 0,95367431640625000000 0,95367431640625000000 0,95367431640625000000 0,00000000000000000000 0,95367431640625000000 0,00000000000000000000

As you can see, there are a couple of lines stating a duration of "0" - which is in fact not possible. If I uncomment the line containing the sleep(0) command, there is no zero-duration.

System-Setup

• PHP 7.0 with FPM
• nginx 1.10.3
• Ubuntu 16.04

I am running the loop on the CLI and calling it via the Browser.

101 items in array is small enough for smart php with it's static optimization tricks and powerful cpu.

If You want to see that 0-s are gone, so generate 1000 items:

for ($i = 1; $i <= 1000; $i++) {
    $haystack[] = substr(md5(microtime()),rand(0,26), 1);
}

P.S. I've checked Your code using both 7.1 and 5.6 so there are big differences: