Method to return to a secondary directory by ID

/*
Secondary directory
Returns a secondary directory by ID, which is used to evenly distribute the content in 36*36 directories.
36 directories are a-z0-9
*/
function IDcat($id){
$p0 = $id % 1296;
$p1 = ceil($p0 / 36)-1;
$p2 = ($ p0-1) % 36 ;
return IDnum($p1).'/'.IDnum($p2).'/';
}
function IDnum($i){
if($i<10){
if ($i==-1){
return 'z';
}else{
return $i;
}
}else{
return chr(87+$i);
}
}
?>