src/MimeMailer/MimeMessage.php
public function setSubject($subject = '', $clear = false)
    {
        if (true===$clear) {
            $this->clear('subject');
        }
        $this->subject = $subject;
        return $this;
    }
public function setSubject($subject = '', $clear = false)
    {
        if (true===$clear) {
            $this->clear('subject');
        }
        $this->subject = $subject;
        return $this;
    }
Set mail object

@param string $subject The subject content
@param bool $clear Clear a set content first ? (default is to append a content)
@return self