Problemen met php contact formulier

<!-- Contact section -->
<section id="contact" class="contact content-section no-bottom-pad">
<div class="container">
<div class="row text-center">
<div class="col-md-12">
<h2>Contact</h2>
<h3 class="caption gray">Neem gerust contact met ons op als je denkt dat wij je ergens mee kunnen helpen.</h3>
</div><!-- /.col-md-12 -->

</div><!-- /.row -->
</div><!-- /.container -->

<div class="container">
<div class="row form-container">

<div class="col-md-8 contact-form">
<h3>Laat een bericht achter</h3>
<form class="ajax-form" id="contactForm" method="post" action="assets/php/contact.php">
<div class="form-group">
<input type="text" class="form-control" id="name" name="name" placeholder="Jouw naam..." value="" required>
</div>
<div class="form-group">
<input type="email" class="form-control" id="email" name="email" placeholder="Jouw email..." value="" required>
</div>
<div class="form-group">
<input type="phone" class="form-control" id="phone" name="phone" placeholder="Jouw telefoonnummer..." value="" required>
</div>
<div class="form-group">
<textarea class="form-control" rows="4" name="message" placeholder="Jouw bericht..." required></textarea>
</div>
<div class="form-group">
<button type="submit" name="submit" class="btn btn-default"><i class="fa fa-paper-plane fa-fw"></i>Verzenden</button>
</div>
</form>
</div><!-- /.contact-form -->

<?php
header('Content-type: application/json');

if($_POST)
{
$to_email = "[email protected]"; //Recipient email, Replace with own email here

//check if its an ajax request, exit if not
if(!isset($_SERVER['HTTP_X_REQUESTED_WITH']) AND strtolower($_SERVER['HTTP_X_REQUESTED_WITH']) != 'xmlhttprequest') {

$output = json_encode(array( //create JSON data
'type'=>'error',
'text' => 'Sorry Request must be Ajax POST'
));
die($output); //exit script outputting json data
}

//Sanitize input data using PHP filter_var().
$user_name = filter_var($_POST["name"], FILTER_SANITIZE_STRING);
$user_email = filter_var($_POST["email"], FILTER_SANITIZE_EMAIL);
$phone_number = filter_var($_POST["phone"], FILTER_SANITIZE_NUMBER_INT);
$message = filter_var($_POST["message"], FILTER_SANITIZE_STRING);

//additional php validation
if(strlen($user_name)<4){ // If length is less than 4 it will output JSON error.
$output = json_encode(array('type'=>'error', 'text' => 'Naam is te kort of leeg'));
die($output);
}

if(!filter_var($user_email, FILTER_VALIDATE_EMAIL)){ //email validation
$output = json_encode(array('type'=>'error', 'text' => 'Vul alsjeblieft een geldig email in'));
die($output);
}


if(!filter_var($phone_number, FILTER_SANITIZE_NUMBER_FLOAT)){ //check for valid numbers in phone number field
$output = json_encode(array('type'=>'error', 'text' => 'Voer alleen cijfers in'));
die($output);
}

if(strlen($message)<3){ //check emtpy message
$output = json_encode(array('type'=>'error', 'text' => 'Sorry, een te kort bericht. Vul iets in'));
die($output);
}

//email subject
$subject ='Bericht via de website Force 10';

//email body
$message_body = $message."\r\n\r\n-".$user_name."\r\n\r\nEmail : ".$user_email."\r\nPhone Number : ". $phone_number ;

//proceed with PHP email.
$headers = 'From: '.$user_name.'<'.$user_email.'>'."\r\n" .
'Reply-To: '.$user_name.'<'.$user_email.'>' . "\r\n" .
'X-Mailer: PHP/' . phpversion();

$send_mail = mail($to_email, $subject, $message_body, $headers);

if(!$send_mail)
{
//If mail couldn't be sent output error. Check your PHP email configuration (if it ever happens)
$output = json_encode(array('type'=>'error', 'text' => 'Could not send mail! Please check your PHP mail configuration.'));
die($output);
}else{
$output = json_encode(array('type'=>'success', 'text' => 'Hi '.$user_name .', Bedankt voor je bericht. We zullen zsm contact opnemen.'));
die($output);
}
}


?>