<select size="1" id="template" name="template">
        <option selected> Содержание письма </option>
        <option value="1" > Приветствие </option>
        <option value="2" > Оповещание </option>
        <option value="3" > Предупреждение </option>
    </select>
    <br>
    <label for="var1"> opt1: </label><input id="var1" type="text">
    <br>
    <label for="var2"> opt2: </label><input id="var2" type="text">
    <br>
    <label for="var3"> opt3: </label><input id="var3" type="text">
    <br>
    <input id="preveiw" type="submit" name="preview" value="Предпросмотр">
    <input type="submit" name="send" value="Отправить">
</form>

<?php 
require_once 'config.php';
require DIR_CLASSES . 'MailgunLetter.php';
require DIR_CLASSES . 'MailgunMailer.php';
$mailer = new MailgunMailer('url', 'key');
if (isset($_POST)) {
    $obj = $mailer->create()->to('me <*****@*****.**>')->subject('test')->setIsHtml()->setTemplate('test1')->setLayout('test2')->addInlineImages([DIR_ROOT . 'img/someotherimg.png', DIR_ROOT . 'unnamed.png', DIR_ROOT . 'img/img.png'])->addAttachments([])->setVariables(['arr' => 'first', 'arr2' => 'second']);
    if (isset($_POST['preview'])) {
        $obj->is_preview();
    } elseif (isset($_POST['send'])) {
        $obj->send();
    }
}
 public function send()
 {
     $this->mailer->sendRequest($this->draw(), $this->is_preview);
 }