Description:
------------
I serialized a SplObjectStorage instance containing several objects.  After unserialization, the SplObjectStorage object contained nothing.

It doesn't say in the SPL documentation that SplObjectStorage cannot be serialized.

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

class test { }

$storage = new SplObjectStorage();

for( $i=0; $i<5; $i++ ) {
     $test = new test();
     $test->example = 'testing'.$i;
     $storage->attach( $test );
}

echo "Before: ".count($storage)."<br>";

$str = serialize($storage);

$storage2 = unserialize( $str );

echo "After: ".count($storage2)."<br>";

?>

Expected result:
----------------
Before: 5
After: 5

Actual result:
--------------
Before: 5
After: 0