Contact form email sending in PHP

contact.php

<?php

// If there were *no* submission errors:
if ($err === 0) {
    if (function_exists('htmlspecialchars_decode')) {
        $subject = htmlspecialchars_decode($subject);
        $message = htmlspecialchars_decode($message);
    }

    $message .= "\r\nPhoneNumber: " . $phone;

    $headers = "Content-type: text/plain; charset=UTF-8\r\n";
    $headers .= "From: " . $name . "<" . $email . ">\r\n";
    $headers .= "Reply-To: $email" . "\r\n";
    mail(MAILTO, SUBJECT_PREFIX . " - " . $subject, $message, $headers);

    // Return the 'noscript' response.
    if (empty($_POST['js'])) {
        readfile(CONFIRM_PAGE);
    }
    // Return the Ajax response.
    else {
        echo 'OK:' . CONFIRM_PAGE;
    }
}

// There was a submission error:
else {
    // Return the 'noscript' response.
    if (empty($_POST['js'])) {
        echo $html_error_page_header . $err . $html_error_page_footer;
    }
    // Return the Ajax response.
    else {
        echo $err;
    }
}
?>