mail script

<?php
    error_reporting(-1);
    ini_set('display_errors', 'On');

    function check_input($data) : string {
        $data = trim($data);
        $data = stripslashes($data);
        $data = htmlspecialchars($data);
        return $data;
    }

    function createHTMLMessage($naam, $email, $phone, $msg, $chkArr) : string {
        $message = '<table>';
        $message .=     '<tr>';
        $message .=         '<td><b>Naam:</b></td>';
        $message .=         '<td>' . $naam . '</td>';
        $message .=     '</tr>';
        $message .=     '<tr>';
        $message .=         '<td><b>Email:</b></td>';
        $message .=         '<td>' . $email . '</td>';
        $message .=     '</tr>';
        if ($phone) {
            $message .=     '<tr>';
            $message .=         '<td><b>Telefoon:</b></td>';
            $message .=         '<td>' . $phone . '</td>';
            $message .=     '</tr>';
        }
        if ($chkArr) {
            $via = '';
            foreach ($chkArr as $item) {
                $via .= $item . '\n';
            }
            $message .=  '<tr>';
            $message .=         '<td><b>Animagie leren kennen via:</b></td>';
            $message .=         '<td>' . $via . '</td>';
            $message .=  '</tr>';
        }
        $message .= '</table>';
        $message .= '<hr />';
        $message .= $msg;

        return $message;
    }

    function createNonHTMLMessage($naam, $email, $phone, $msg, $chkArr) : string {
        $message = "Naam:\t\t\t" . $naam . "\n";
        $message .= "Email:\t\t\t" . $email . "\n";
        if ($phone) {
            $message .= "Telefoon:\t\t\t" . $phone . "\n";
        }
        if ($chkArr) {
            $via = '';
            foreach ($chkArr as $item) {
                $via .= $item . '\n';
            }
        }
        $message .= "Bericht:\n\n" . $msg . "\n";

        return $message;
    }

    function sendMail($naam, $email, $phone, $subj, $msg, $chkArr) {
        require 'PHPMailer/PHPMailerAutoload.php';
        $TO = ' ... '; //TODO

        $mail = new PHPMailer(true);

        try {
            $mail->setLanguage('nl');
            $mail->isSMTP();
            $mail->Host = 'smtp.gmail.com'; //TODO
            $mail->SMTPAuth = true;
            $mail->Username = ' ... '; //TODO
            $mail->Password = ' ... '; //TODO
            $mail->SMTPSecure = 'tls';
            $mail->Port = 587;

            $mail->setFrom($email, $naam);
            $mail->addAddress($TO, 'Animagie');
            $mail->isHTML(true);

            $mail->Subject = 'Animagie: ' . $subj;
            $mail->Body = createHTMLMessage($naam, $email, $phone, $msg, $chkArr);
            $mail->AltBody = createNonHTMLMessage($naam, $email, $phone, $msg, $chkArr);

            if (!$mail->send()) {
                $error = "Bericht kon niet worden verstuurd: " . $mail->ErrorInfo;
                echo $error;
                exit;
            } else {
                //TODO: return to success page
                echo 'Bericht succesvol verstuurd!';
                exit;
            }
        } catch (phpmailerException $ex) {
            echo $ex->getMessage();
            exit;
        } catch (Exception $ex) {
            echo $ex->getMessage();
            exit;
        }
    }

    if ($_SERVER['REQUEST_METHOD'] === 'POST') {

            /*some input validation */
            sendMail($naam, $email, $phone, $subj, $msg, $chkArr);
    }
?>