Filp a random bit in a string with PHP

flip.php

<?php
function flip_random_bit(string $str)
{
    $charIndex = random_int(0, strlen($str) - 1);
    $bitIndex = random_int(0, 8);
    $str[$charIndex] = chr(ord($str[$charIndex]) ^ 1 << $bitIndex);

    return $str;
}