how to randomize an json array php

<?php
header('Content-type: application/json');
$arr = array('success' => true, 'key' => "random", 'valid_until' => "2020-12-23 23:38:53", 'pid' => LssBot, 'token' => "QmBLOJmCRWe+jdVyKKSUJaxtMDPloLFyuQ490USkRQSFxEiBMjzsjd+34IoxryTVw7OZH8p7zY4vdewkyML5oLe75VchNTaWTS6OUPCDaPEM+nevz\/3n9NBv+v2mm\/C2YamlApCPhR+vJm376MFKYuNZz1CqUgt5LhEG3lgAOHHlqydS5tBc1L\/0VrnHuvWzClCUcrQ31s9Igo2C2+g\/LosARCqSIZB+1x9nOiFhiSKYKPqzEbHXwZybEhX7ztQ63U0bQEG6k0qHl+PObkR8ap7qHXDjrj2Dh5BVOe5jVQZC19qt8NQ9qlnxfqOqPIKaC\/2fLrWCJAGpfxanltNuXW+HXAnulrTDKrO430tbwR0=", 'r' => "b22831623da8b1e20a76376f057455da", 'xt' => "2");

echo json_encode($arr);

?>

i want to randomize the key value 'key' => "random" as 32 random numbers