Description:
------------
When implementing \Serializable a nested serialize() call will cause the end result to be messed up.

Nested serialize() calls often occur when extending classes and overwriting serialize with additional fields, so this is not unusual. Our test script simulates this by nesting serialize().

The result still break, if the array with the referenced objects is not inside the nested serialize(), but instead any additional property is serialized with a nested call (not in the test script).

Test script:
---------------
$role1 = new \stdClass();
$role1->name = 'role1';
$role2 = new \stdClass();
$role2->name = 'role2';
class group implements \Serializable {
  private $roles;
  public function __construct(array $roles) {
    $this->roles = $roles;
  }
  public function serialize() {
    return serialize([serialize($this->roles)]);
  }
  public function unserialize($serialized) {
    $this->roles = unserialize(unserialize($serialized)[0]);
  }
}
$group1 = new \group([$role1, $role2]);
$group2 = new \group([$role1, $role2]);
$serialized = serialize([$group1, $group2]);
echo "$serialized\n";

Expected result:
----------------
a:2:{i:0;C:5:"group":116:{a:1:{i:0;s:98:"a:2:{i:0;O:8:"stdClass":1:{s:4:"name";s:5:"role1";}i:1;O:8:"stdClass":1:{s:4:"name";s:5:"role2";}}";}}i:1;C:5:"group":116:{a:1:{i:0;s:98:"a:2:{i:0;O:8:"stdClass":1:{s:4:"name";s:5:"role1";}i1;O:8:"stdClass":1:{s:4:"name";s:5:"role2";}}";}}}

Actual result:
--------------
a:2:{i:0;C:5:"group":116:{a:1:{i:0;s:98:"a:2:{i:0;O:8:"stdClass":1:{s:4:"name";s:5:"role1";}i:1;O:8:"stdClass":1:{s:4:"name";s:5:"role2";}}";}}i:1;C:5:"group":40:{a:1:{i:0;s:22:"a:2:{i:0;r:4;i:1;r:6;}";}}}