Description:
------------
class a{}
$object = new a();
$object2 = spl_get_object_by_hash(spl_object_hash($object));
var_dump($object === $object2); // true (same object)

NOTE: When using spl_get_object_by_hash, you must know that $object must be alive 
else function can return another object.

Primary usage example in test script:

Test script:
---------------
class a{
  protected $_b;
  public function getB(){
     return spl_get_object_by_hash($this->_b);
  }
  public function doSomething(){
    $b = $this->getB();
    // here I can use $b !
  }
  public function __construct($b){
     $this->_b = spl_object_hash($b);
  }
}
class b{
  protected $_a;
  public function getA(){
     return $this->_a;
  }
  public function __construct(){
     $this->_a = new a($this);
  }
  public function __destruct(){
    echo 'destroyed';
  }
}
$b = new b();
$b->getA()->doSomething();
unset($b); // destroyed  !! this IS NOT POSSIBLE without spl_get_object_by_hash()