MailController.php

<?php
// Package symfony/mailer must be installed (`composer require symfony/mailer`)
require_once $_SERVER['DOCUMENT_ROOT'] . '/../vendor/autoload.php';

use Symfony\Component\Mailer\Mailer;
use Symfony\Component\Mailer\Transport\SendmailTransport;
use Symfony\Component\Mime\Email;

class MailController
{
	private $mailer;

	public function __construct()
	{
		$sendmail_path = ini_get('sendmail_path') ?: null;
		if ($sendmail_path) $sendmail_path .= " -t"; // Extract recipients from message headers.

		$transport = new SendmailTransport($sendmail_path);
		$this->mailer = new Mailer($transport);
	}

	public function actionSendCallback()
	{
		$_POST = filter_input_array(INPUT_POST, FILTER_SANITIZE_STRING);

		$email = (new Email())
			->from(static::getFromEmail())
			->to('admin@localhost')
			->subject('Request callback.')
			->text("Name: {$_POST['name']},\n Email: {$_POST['email']}")
			->html("Name: <strong>{$_POST['name']}</strong>,<br> Email: <strong>{$_POST['email']}</strong>");

		$this->mailer->send($email);
	}

	public static function getFromEmail()
	{
		return "no-replay@{$_SERVER['SERVER_NAME']}";
	}
}

try {
	$action = 'action' . ($_GET['action'] ?? '');

	$mailController = new MailController();
	if (!method_exists($mailController, $action)) throw new \Exception('Disallowed action.');

	$mailController->$action();
} catch (\Exception $e) {
	header($_SERVER['SERVER_PROTOCOL'] . ' 500 Internal Server Error', true, 500);
}