Description:
------------
When one of the participants of a cyclic reference holds a reference to a DateTime instance, the GC seems to be unable to do its job.

NOTE: Even without a DateTime reference memory keeps increasing slowly but steadily. This is not the case as soon as either $a->b = $b or $b->a = $a is commented out, i.e. the cyclic reference is removed. So even though the GC seems to work almost perfectly (without a DateTime reference), a small leak remains.

The leakage also occurs with PDO, however not with other php internal objects.

Reproduce code:
---------------
class A {
  public $b;
  public $ref;
  function __construct() {
    $this->ref = new DateTime; // "large" leak. comment out for small leak.
  }
}

class B {
  public $a;
}


for ($i=1; $i<=200000; ++$i) {
  $a = new A;
  $b = new B;
  $a->b = $b; // comment out to avoid any leakage, with or without DateTime, doesnt matter.
  $b->a = $a; // comment out to avoid any leakage, with or without DateTime, doesnt matter.

  if ($i % 10000 == 0) {
    gc_collect_cycles();
    printf('----- Memory usage after %d iterations: %2.2f MB' .PHP_EOL, $i, memory_get_usage() / 1024 / 1024);
  }
}


Expected result:
----------------
----- Memory usage after 10000 iterations: 0.79 MB
----- Memory usage after 20000 iterations: 0.79 MB
----- Memory usage after 30000 iterations: 0.79 MB
----- Memory usage after 40000 iterations: 0.79 MB
----- Memory usage after 50000 iterations: 0.79 MB
----- Memory usage after 60000 iterations: 0.79 MB
----- Memory usage after 70000 iterations: 0.79 MB
----- Memory usage after 80000 iterations: 0.79 MB
----- Memory usage after 90000 iterations: 0.79 MB
----- Memory usage after 100000 iterations: 0.79 MB



Actual result:
--------------
----- Memory usage after 10000 iterations: 4.53 MB
----- Memory usage after 20000 iterations: 8.38 MB
----- Memory usage after 30000 iterations: 12.15 MB
----- Memory usage after 40000 iterations: 15.90 MB
----- Memory usage after 50000 iterations: 19.65 MB
----- Memory usage after 60000 iterations: 23.40 MB
----- Memory usage after 70000 iterations: 27.15 MB
----- Memory usage after 80000 iterations: 30.90 MB
----- Memory usage after 90000 iterations: 34.65 MB
----- Memory usage after 100000 iterations: 38.41 MB