Description:
------------
From http://stackoverflow.com/questions/43162831/zend-mm-heap-corrupted-with-php-7-1.

If numUsed occupancy is <= 3/4 array_rand() compacts the array prior to sampling. This is done on a potentially shared array.

This may lead to SHM corruption, for example:

$array = [1 => 1, 2 => 2];
var_dump(array_rand($array));

Crashes on opcache.

It can also have other side effects:

<?php

$array = range(0, 100);
for ($i = 0; $i < 50; $i++) {
    unset($array[$i]);
}

foreach ($array as $x) {
    var_dump($x);
    if ($x == 55) {
        array_rand($array, 1);
    }
}

Here the array is resized during the loop, leaving a dangling pointer.