测试静态功能

Question

我是测试的新手。我正在使用PHPUnit Framework。我读到了关于静态函数的嘲弄和存根，但即使这些概念对我来说也是新的。我想对以下功能进行单元测试

class Company implements ifOrm {
    public $id              = null;
    public $externalId      = null;
    public $name            = null;
    public $created         = null;
    public $modified            = null;
    public $state = null;
    private $arrService = array();

    public function __construct($externalId, $name, $state) {
        $this->externalId = $externalId;
        $this->name = $name;        
        $this->state = $state;      
    }

    public static function factoryFromObject($obj) {
        $c = new Company($obj->external_id, $obj->name, $obj->state);
        // Tag it
        $c->id          = $obj->id;
        $c->created     = $obj->created;
        $c->modified   = $obj->modified;

        return $c;
    }
}

到目前为止我所做的是以下

 class CompanyTest extends PHPUnit_Framework_TestCase
{
private $Company;

public function testFactoryFromObject()
{
    $this->Company = new Company( 004 ,'CompanyName','State');

    $returnObject = $this->Company->factoryFromObject($this->Company);

    $expectedId = $this->Company->id;
    $expectedCreated = $this->Company->created;
    $expectedModified = $this->Company->modified; 

    $actualId = $this->returnObject->id;
    $actualCreated = $this->returnObject->created;
    $actualModified = $this->returnObject->modified;

    assertEquals($expectedId, $actualId);
    assertEquals($expectedCreated, $actualCreated);
    assertEquals($expectedModified, $actualModified);       

   }
}

我收到以下错误：

Undefined property: Company::$external_id

我如何测试这个功能，任何帮助都会很棒，谢谢。