Segmentation fault

serialize.php

<?php
class Permission implements \Serializable {
    protected $id = 3;
    public function serialize()
    {
        return serialize(array($this->id));
    }
    public function unserialize($serialized)
    {
        list($this->id) = unserialize($serialized);
    }

}

class UserPermission implements \Serializable {
    public $permission;
    public $webshop;

    public function serialize()
    {
        return serialize(array($this->webshop, $this->permission));
    }
    public function unserialize($serialized)
    {
        list($this->webshop, $this->permission) = unserialize($serialized);
    }
}

class Webshop  implements \Serializable {
    protected $id = 13;
    public function serialize()
    {
        return serialize(array($this->id));
    }
    public function unserialize($serialized)
    {
        list($this->id) = unserialize($serialized);
    }

}

class AbstractToken  implements \Serializable {
    public $roles;

    public function serialize()
    {
        return serialize(array($this->roles));
    }
    public function unserialize($serialized)
    {
        list($this->roles) = unserialize($serialized);
    }

}
class UsernamePasswordToken extends AbstractToken {
    private $credentials = null;
    private $providerKey = null;

    public function serialize()
    {
        return serialize(array($this->credentials, $this->providerKey, parent::serialize()));
    }

    public function unserialize($str)
    {
        list($this->credentials, $this->providerKey, $parentStr) = unserialize($str);
        parent::unserialize($parentStr);
    }
}

$token = new UsernamePasswordToken();
$webshop = new Webshop;
$permission = new Permission;
$roles = array();
for ($i = 0; $i < 2; $i++) {
    $roles[$i] = new UserPermission();
    $roles[$i]->webshop = $webshop;
    $roles[$i]->permission = $permission;
}
$token->roles = $roles;
var_dump(unserialize(serialize($token)));

Cloning object in UserPermission::serialize helps:

public function serialize()
{
    $this->webshop = clone $this->webshop;
    $this->permission = clone $this->permission;
    return serialize(array($this->webshop, $this->permission));
}

Avoiding parent::(un)serialize() too