How can you use javascript alert in php

<?php


require 'vendor/autoload.php';
date_default_timezone_set('Africa/Harare');

use Monolog\Logger; 
use Monolog\Handler\StreamHandler;

function phpAlert($msg_error) {
    echo '<script type="text/javascript">alert("' . $msg_error . '")</script>';

} 


//$log = new Logger('name');
//$log->pushHandler(new StreamHandler('app.txt', Logger::WARNING));
//$log->addWarning('Foo');

$app = new \Slim\Slim( array(
  'view' => new \Slim\Views\Twig()
));

$view = $app->view();
$view->parserOptions = array(
    'debug' => true
);
$view->parserExtensions = array(
    new \Slim\Views\TwigExtension(),
);

$app->get('/', function() use($app){
  $app->render('home.twig');
})->name('home');

$app->get('/', function() use($app){
  $app->render('about.twig');
})->name('about');

$app->get('/contact', function() use($app){
  $app->render('contact.twig');
})->name('contact');

$app->post('/contact', function() use($app){
  $name= $app->request->post('name');
  $age=$app->request->post('age');
  $email=$app->request->post('email');
  $expected_date=$app->request->post('expected_date');
  $country_to_visit=$app->request->post('country_to_visit');
  $reason_for_visit=$app->request->post('reason_for_visit');
  $payment_method=$app->request->post('payment_method');
  $msg=$app->request->post('msg');

  if(!empty($name) && !empty($email) && !empty($msg)) {
    $cleanName = filter_var($name, FILTER_SANITIZE_STRING);
    $cleanEmail = filter_var($email, FILTER_SANITIZE_EMAIL);
    $cleanMsg = filter_var($msg, FILTER_SANITIZE_STRING);
  } else {
    phpAlert(   "All the fields are needed. Please fill all fields and send again"   );
    die();
    $app->redirect('/contact'); 

  }

  $transport = Swift_MailTransport::newInstance('/usr/sbin/sendmail -bs');
  $mailer = \Swift_Mailer::newInstance($transport);

  $message= \Swift_Message::newInstance();
  $message->setSubject('Email From Our Website');
  $message->setFrom(array($cleanEmail=>$cleanName));
  $message->setTo(array('selesgama@gmal.com'));
  $message->setBody($cleanMsg);

  $result = $mailer->send($message);
  if($result > 2){

    phpAlert(   "Thank you for contacting us. We will get to you soon."   ); 
    die();
    $app->redirect('/');

  } else{
    phpAlert(   "There was an error sending the message. Try again later."   ); 
    die();

  }
    $app->redirect('/contact');
});

$app->run();

?>