Try This one. It will generate id according to your wish.
function id()
{
// add limit
$id_length = 20;
// add any character / digit
$alfa = "abcdefghijklmnopqrstuvwxyz1234567890";
$token = "";
for($i = 1; $i < $id_length; $i ++) {
// generate randomly within given character/digits
@$token .= $alfa[rand(1, strlen($alfa))];
}
return $token;
}