Google\Spreadsheet\Spreadsheet::getId PHP Method

    public function getId()
    {
        return $this->xml->id->__toString();
    }

 public function testGetId()
 {
     $spreadsheet = new Spreadsheet($this->getSimpleXMLElement("spreadsheet"));
     $this->assertEquals($this->serviceUrl . "tFEgU8ywJkkjcZjG", $spreadsheet->getId());
 }