A couple minor fixes: more flexible version numbering; added a CreateDirectory before uninstaller generation (whoops)
<?php
$chars = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789';
for ( $i = 0; $i < 8; $i++ )
echo $chars{ rand(0, strlen($chars)-1) };