- preg_replace_callback: documentation ( source)
- rand: documentation ( source)
<?php
$str = 'Hello_world!';
$chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789/-';
$max = strlen($chars) - 1;
echo preg_replace_callback(
'/.\K/',
fn() => $chars[rand(0, $max)],
$str
);