HTML form showing the php code after submit - php

Question

I have a simple html form what I use in all of my websites in different servers.
In a new server I found a problem I cant solve by my own.
The problem is that after hit the submit button php code doesnt send the email to the server email and doesnt direct to the thankyou.html page. But it does direct to mydomain.com/contactform.php and shows the php code. Firstly I though something wrong with the server configuration, but i didnt find anything. Could anyone give my any advice where to search? Thanks in advance!
PHP code:
<?php
if($_SERVER["REQUEST_METHOD"] === "POST") {}
if (isset($_POST['submit'])) {
$name = $_POST['name'];
$subject = $_POST['subject'];
$mailFrom = $_POST['mail'];
$place = $_POST['place'];
$number = $_POST['number'];
$csoportjelleg = $_POST['csoportjelleg'];
$message = $_POST['message'];
$recaptcha_secret = "secret-key";
$response = file_get_contents("https://www.google.com/recaptcha/api/siteverify?secret=".$recaptcha_secret."&response=".$_POST['g-recaptcha-response']);
$response = json_decode($response, true);
if($response["success"] === true){
$mailTo = "info#serveremail.com";
$headers = "From: ".$mailFrom;
$txt = "Feladó: ".$name."\nInnenjönnek: ".$place."\nLétszám: ".$number."\nCsoportjelleg: ".$csoportjelleg."\n\nÜzenet: ".$message;
mail($mailTo, $subject, $txt, $headers);
header("Location: thankyou");
}else{
header("Location: error");
}
}
?>
HTML code:
<form action="contactform.php" method="post" class="ajanlatkeres-form">
<h2 class="centered-h2">Általános adatok</h2>
<input type="text" name="name" placeholder="Teljes név*" class="feedback-input" required>
<input type="text" name="mail" placeholder="E-mail*" class="feedback-input" required>
<h2 class="centered-h2">További adatok</h2>
<input type="text" name="place" placeholder="Honnan jöttök?" class="feedback-input">
<input type="text" name="number" placeholder="Hány fős a csoport?" class="feedback-input">
<div class="feedback-input-div">
<p class="form-p">Csoport jelleg?</p>
<label class="container">Munkahelyi
<input type="radio" name="csoportjelleg" required>
<span class="checkmark"></span>
</label>
<label class="container">Iskolai
<input type="radio" name="csoportjelleg">
<span class="checkmark"></span>
</label>
<label class="container">Családi
<input type="radio" name="csoportjelleg">
<span class="checkmark"></span>
</label>
<label class="container">Egyéb
<input type="radio" name="csoportjelleg">
<span class="checkmark"></span>
</label>
</div>
<h2 class="centered-h2">Üzenet</h2>
<input type="hidden" name="subject" value="Csoportos ajánlatkérés">
<textarea name="message" placeholder="Ajánlatkérés üzenete:*" class="feedback-input" required></textarea>
<div class="g-recaptcha" id="rcaptcha" data-sitekey="6LdBDf4UAAAAAJ50InC0WKfVep4263x3Bmuz9-60"></div><br/>
<div class="button-form">
<button id="submit_form" class="contactbutton" type="submit" name="submit">Küldés</button>
</div>
UI.: PHP form location: I use thankyou and error without .html extension because there are no extensions in my ULR-s thanks to .htaccess manipulations. In other servers that was absolutly ok.
Updated, thanks kerbh0lz!