Description:
------------
The example says it the best. Reproducible in both 5.1.6 and 4.4.4 (with minor changes), all extensions disabled. Definately related to references, although I'm not sure if the code below is good practice, but I've run into it on more than one occasion. I think it should be freeing the objects when they go out of scope, but the reference counting must be broken somehow. 

Might be related to #22055, but this one's been around for ages.

Reproduce code:
---------------
<?php {
  class A {
    var $ba;
    function A() {
      $this->ba = array();
      for ($i = 0; $i < 100; $i++)
        $this->ba[] = new B($this);
    }
    function __destruct() { // never gets called
      die("Freeing class A\n");
    }
  }

  class B {
    var $a;
    function B($a) { // function B(&$a), and:
      $this->a = $a; // $this->a =& $a; leaks in php 4.4.4
      // remove line above to prevent leak
    }
    function __destruct() { // never gets called either
      die("Freeing class B\n");
    }
  }

  $smu = memory_get_usage();
  while (true) {
    $ba = array();
    for ($i = 0; $i < 100; $i++)
      $ba[] = new A();
    $emu = memory_get_usage();
    echo sprintf("leak %.1f kB\r", ($emu - $smu) / 1024.0);
  }
}?>


Expected result:
----------------
No memory leak.

Actual result:
--------------
Rapidly increasing memory leak.