ob_get_contents
в помощь.<table>
<tr>
<td><?= $variable_1 ?></td>
</tr>
<tr>
<td><?= $variable_2 ?></td>
</tr>
</table>
$mail = new PHPMailer();
$variable_1 = "Привет";
$variable_2 = "Чухча";
ob_start();
require_once("template.php");
$body = ob_get_contents();
ob_end_clean();
.....
$mail->Body = $body;
.....