I've answered my own question.
I've copied the function random_string () in /lib/moodlelib.php so that it doesn't use the characters
I've copied the function random_string () in /lib/moodlelib.php so that it doesn't use the characters
function random_string_cert($length=15) {
$randombytes = random_bytes_emulate($length);
$pool = 'ABCDEFGHJKLMNPQRSTVWXYZ';
$pool .= 'abcdefghijkmnpqrstvwxyz';
$pool .= '123456789';
$poollen = strlen($pool);
$string = '';
for ($i = 0; $i < $length; $i++) {
$rand = ord($randombytes[$i]);
$string .= substr($pool, ($rand%($poollen)), 1);
}
return $string;
}
and then called this new function in /mod/customcert/classes/certificate.php