function getLastIndex(){
    // query+get the last index value
    return lastIndex;
}
function updateLastIndex($id){
    //query+save $id
}
function indexFormula($id){
    return ($id*2)+5 //you can use a more complicated formula or whatever
}
function getNewLink(){
    $id = getLastIndex();
    $id = indexFormula($id);
    updateLastIndex($id);
    //you can use base64 or whatever here
    return $id;
}