Description:
------------
Apache crashes and CLI segfault when serializing objects which have circular references and implements the Serializable interface.
Whitout the interface, the serialization and deserialization occurs without any problem.

Reproduce code:
---------------
<?php

class A implements Serializable {

  private $b;

  public function setB($b) {
    $this->b = $b;
  }
  
  public function serialize() {
    return serialize($this->b);
  }
  
  public function unserialize($s) {
    $this->b = unserialize($s);
  }

}

class B {

  private $a;

  public function __construct($a) {
    $a->setB($this);
    $this->a = $a;
  }
}


$a = new A();
$b = new B($a);

var_dump(serialize($b));
?>


Expected result:
----------------
The string of the serialized object.

Actual result:
--------------
Apache crash or cli segfault.