- array_slice: documentation ( source)
- implode: documentation ( source)
- shuffle: documentation ( source)
- str_split: documentation ( source)
<?php
$arr = str_split("ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz1234567890", 1);
shuffle($arr);
echo implode('', array_slice($arr, 0, 16));