How do I make PHP mail form functional? [closed] – Blancer.com Tutorials and projects

I have like two files, both are in the same and correct folder:

index.php and contactform.php

The form appears, but I want to make if fully functional, so it actually sends an email to my email address. Can someone review the code and tell me what have I done wrong and how can I make it work, so It sends an email to [email protected]?

index.php:

<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <link href="https://fonts.googleapis.com/css2?family=Poppins:wght@400;600&display=swap" rel="stylesheet">
    <link="stylesheet" href="index.css"> <!-- zde doplnit odkaz na CSSka -->    
</head>
<body>
    <p>SEND E-MAIL</p>  
    <form class="contact-form" action="contactform.php" method="post">
        <input type="text" name="name" placeholder="Full name">
        <input type="text" name="mail" placeholder="Your-email">
        <input type="text" name="subject" placeholder="Subject">
        <textarea name="message" placeholder="Message"></textarea>
        <button type="submit" name="submit">SEND MAIL</button>
    </form>
</body>
</html>

contactform.php:

<?php

if (isset($_POST['submit'])) {
    $name = $_POST['name'];
    $subject = $_POST['subject'];
    $mailFrom = $_POST['mail'];
    $message = $_POST['message'];

    $mailTo = "[email protected]";
    $headers = "From: ".$mailFrom;
    $txt = "You have received an e-mail from ".$name.".nn".$message;


    mail($mailTo, $subject, $txt, $headers);
    header("Location: index.php?mailsend")
}

?>