How to attach a file to an email without saving it to disk?

Question

Yii

Tell me how to attach a file to a letter using ->attach() without saving it to disk?
There is a file which needs to be hooked to the letter. But attach only accepts the path to the saved file, how to get around this?

$pdf=new \FPDF();
            $pdf->AddPage();
            $pdf->Image($envelope,60,30,90,0);

Yii::$app->mailer->compose()
                ->setTo('@gmail.com')
                ->setFrom(['@gmail.com' => Yii::$app->params['senderName']])
                ->setReplyTo([$this->email => $this->fromPerson])
                ->setSubject("Ваш конверт")
                ->setTextBody("Вам отправил письмо {$this->fromPerson} проживающий по адресу {$this->fromPlace}.")
                ->setBody($this->pdf) // нужно прикрепить pdf
                ->send();

            return true;