PHP - class intern static registry array(&$this) not working

Question

Greetings dear Community,
I am trying to write a class which keeps track of all instances internally via a static array of references to each object:

# MyObject.php
class MyObject{
  public static $_register = array();
  public        $_id    = -1;
  public        $_value = '';

  function __construct( $value ){
    $this->_value = $value;
    $this->_id    = count( self::$_register );

    self::$_register[ $this->_id ] =& $this;  // I also tried '= &$this'
  }

  function foo(){                                  // outputs 'id : value'
    echo $this->_id.' : ' . $this->_value . '<br />';
  }
}

But when I try to alter $_register in my main script:

#main.php
require_once( 'MyObject.php' );

$test = new MyObject( 'hihi' );
$test->foo();                                     // outputs '0 : hihi'

MyObject::$_register[0] = NULL;

$test->foo();                                     // still outputs '0 : hihi'

It still outputs '0 : hihi'. A var_dump( MyObject::$_register[0] ) showed that it's set to NULL, but the should-be-referenced MyObject still points to the same Instance.

I am trying to understand references in PHP and this behaviour was unexpected for me. Could someone please explain it.

Thanks in advice

P.S: Before I tried a little C++/Qt and the difference in usage of references/pointers is making me dizzy.

Answer 1

That's because with unset you'll destroy only reference to object, but not object itself. This may seems odd, but unsetting a reference is treated as destroying reference, not entity to which that reference is pointing.

In PHP, however, reference is NOT a pointer. The big difference is shown in PHP references tutorial. So in common case - no, you can not unset an entity with given reference to it. Remember: PHP references have nothing to do with C-pointers.