//generate PDF file for bill and send it in email
public function sendInvoiceInEmail($register_no, $html_string, $guest_email_id)
{
    //create mailer class
    $mail = new PHPMailer;
    //remove old invoice file if exist at public/doc
    // here i have used my path you have to use your path
    if(file_exists(__DIR__.'/../../public/docs/invoice.pdf')){
        unlink(__DIR__.'/../../public/docs/invoice.pdf');
    }

    // You can pass a filename, a HTML string, an URL or an options array to the constructor
    $pdf = new Pdf([
        'commandOptions' => [
            'useExec' => false,
            'escapeArgs' => false,
            'procOptions' => array(
            // This will bypass the cmd.exe which seems to be recommended on Windows
            'bypass_shell' => true,
            // Also worth a try if you get unexplainable errors
            'suppress_errors' => true,
            ),
        ],
    ]);
    $globalOptions = array(
        'no-outline'
    );

    $pdf->setOptions($globalOptions);

    $pdf->addPage($html_string);
    //for Windows in my system i have setup wkhtmltopdf here
    $pdf->binary = '"C:/Program Files/wkhtmltopdf/bin/wkhtmltopdf.exe"';
    // for linux when you will host you have to setp on linux abd comment windows path and uncomment below line
    //$pdf->binary = '/home/username/wkhtmltox/bin/wkhtmltopdf';
    if (!$pdf->saveAs(__DIR__.'/../../public/docs/invoice.pdf')) {
        throw new Exception('Could not create PDF: '.$pdf->getError());
    }
    //now send email and attach invoice.pdf file from public/doc/invoice.pdf



    //$mail->SMTPDebug = 3;                               // Enable verbose debug output
    $mail->isSMTP();                                      // Set mailer to use SMTP
    $mail->Host = 'hostname';  // Specify main and backup SMTP servers
    $mail->SMTPAuth = true;                               // Enable SMTP authentication
    $mail->Username = 'username';                 // SMTP username
    $mail->Password = 'password';                           // SMTP password
    $mail->SMTPSecure = 'tls';                            // Enable TLS encryption, `ssl` also accepted
    $mail->Port = 587;                                    // TCP port to connect to
    $mail->setFrom('from_email_address', 'From Name',FALSE);
    $mail->addAddress('to_email_address');     // Add a recipient

    $mail->addAttachment(__DIR__.'/../../public/docs/invoice.pdf');         // Add attachments

    $mail->isHTML(true);                                  // Set email format to HTML

    $mail->Subject = 'Subject';
    $mail->Body    = 'email message body';

    $is_email_sent = false;
    if(!$mail->send()) {
        // echo 'Message could not be sent.';
        // echo 'Mailer Error: ' . $mail->ErrorInfo;
        $is_email_sent = false;
    } else {
        // echo 'Message has been sent';
        $is_email_sent = true;
    }

    return true;
}