<?php
declare(strict_types=1);

$str = 'Hello World';

$shm_key = ftok(__FILE__, 'p');

$shm_id1 = shmop_open($shm_key, 'c', 0644, strlen($str));
shmop_delete($shm_id1);

$shm_id2 = shmop_open($shm_key, 'c', 0644, strlen($str) + 10);
                                                                   // ^ -- This is the difference

if ($shm_id2) {
    shmop_write($shm_id2, $str, 0);
}
else
    throw new RuntimeException("Couldn't create shared memory segment.");