gistfile1.php

<?php

$a = $b = ['a' => 'a',
           'b' => new Datetime('10 minutes ago'),
           'c' => ['a' => 'a',
                   'b' => new Datetime('20 minutes ago')]];

$b['b'] = new Datetime('+10 minutes');
//$b['c']['b'] = new Datetime('+20 minutes');

$f = function ($a, $b) use (&$f) {
        if (gettype($a) !== gettype($b)) {
            return 1;
        }

        if (is_object($a)) {
            var_dump($a, $b, spl_object_hash($a), spl_object_hash($b), spl_object_hash($a) === spl_object_hash($b));
            return spl_object_hash($a) === spl_object_hash($b) ? 0 : 1;
        }

        if (is_array($a)) { var_dump('recursive');
            return [] === array_udiff($b, $a, $f) ? 0 : 1;
        }

        return $a === $b ? 0 : 1;
    };

$c = array_udiff($b, $a, $f);
// var_dump($c);

output

// -- var_dump on the spl_object_hash

class DateTime#3 (3) {
  public $date =>
  string(19) "2013-10-25 10:03:59"
  public $timezone_type =>
  int(3)
  public $timezone =>
  string(3) "UTC"
}
class DateTime#1 (3) {
  public $date =>
  string(19) "2013-10-25 09:43:59"
  public $timezone_type =>
  int(3)
  public $timezone =>
  string(3) "UTC"
}
string(32) "000000000603f01b00000000119276c7"
string(32) "000000000603f01900000000119276c7"
bool(false)

// -- var dump on the result

array(2) {
  'b' =>
  class DateTime#3 (3) {
    public $date =>
    string(19) "2013-10-25 10:16:22"
    public $timezone_type =>
    int(3)
    public $timezone =>
    string(3) "UTC"
  }
  'c' => // WTF ? this thing didn't change like AT ALL...
  array(2) {
    'a' =>
    string(1) "a"
    'b' =>
    class DateTime#2 (3) {
      public $date =>
      string(19) "2013-10-25 09:46:22"
      public $timezone_type =>
      int(3)
      public $timezone =>
      string(3) "UTC"
    }
  }
}