Mailgun\Messages\MessageBuilder::setSubject PHP Method

    public function setSubject($subject = '')
    {
        if ($subject == null || $subject == '') {
            $subject = ' ';
        }
        $this->message['subject'] = $subject;
        return $this->message['subject'];
    }

 /**
  * @param array $message
  *
  * @return \Mailgun\Messages\MessageBuilder
  * @throws \Mailgun\Messages\Exceptions\TooManyParameters
  */
 protected function createMailgunMessage(array $message)
 {
     $builder = new MessageBuilder();
     $builder->setFromAddress($message['from']);
     $builder->setSubject($message['subject']);
     foreach ($message['to'] as $to) {
         $builder->addToRecipient($to);
     }
     foreach ($message['cc'] as $cc) {
         $builder->addCcRecipient($cc);
     }
     foreach ($message['bcc'] as $bcc) {
         $builder->addBccRecipient($bcc);
     }
     if ($message['html']) {
         $builder->setHtmlBody($message['html']);
     }
     if ($message['text']) {
         $builder->setTextBody($message['text']);
     }
     return $builder;
 }