Description:
------------
the test code run ok less than php 7.1.6 .

error on php 7.1.7

Test script:
---------------
$i = 0;
while ($i++ < 3) {
    $pid = pcntl_fork(); //创建子进程
    if ($pid == 0) {
        echo "$i.";
        sleep(1);
        exit;
    }
    pcntl_wait($status);
}

Expected result:
----------------
1.2.3.

Actual result:
--------------
1.