public function setPrivateKey(string $serverKey): self
    {
        $this->serverKeyFingerprint = $this->import($serverKey);
        return $this;
    }
public function setPrivateKey(string $serverKey): self
    {
        $this->serverKeyFingerprint = $this->import($serverKey);
        return $this;
    }
public function setPrivateKey ( string $ serverKey ) : self { $ this -> serverKeyFingerprint = $ this -> import ( $ serverKey ) ; return $ this ; }
Sets the private key for signing.

@param string $serverKey
@return self
@throws GPGMailerException
Sets the private key for signing .