<?php
function uri_to_array($uri){
  $result = array();
  
  parse_str(substr($uri, strpos($uri, '?') + 1), $result);
  list($result['user'], $result['page']) = explode('/', trim($uri, '/'));
  
  return $result;
}

print_r(
  uri_to_array('/admin/stock/?sub_page=products&action=add')
);

Array
(
    [sub_page] => products
    [action] => add
    [page] => stock
    [user] => admin
)