[Dragonsys]

well, the folder structure will always be 1 digit. So you know user #111's attachments will be in folder attachments/1/1/1/files. user #112 will be in attachments/1/1/2/files. user 345 will be in attachments/3/4/5/files etc

user #3000 = attachments/3/0/0/0/files
user #30000 = attachments/3/0/0/0/0/files

you can see that the folders stack.

Try something like this (stolen from Stack Exchange):

PHP Code:

puteveryXcharacters($userid,"/",1);

function puteveryXcharacters($str,$wha,$cnt) {
  $strip = false;
  if (strlen($str) % $cnt == 0) {
    $strip = true;
  }
  $tmp = preg_replace('/(.{'.$cnt.'})/',"$1$wha", $str);
  if ($strip) {
    $tmp = substr($tmp,0,-1);
  }
  return $tmp;
} 


if userid is 123 this should return 1/2/3

NOTE: This has not been tested by me, so use with caution.