mail($email, $subject, $autoreply);
    mail($default, $subject, $autoreply, $naam, $email, $telefoon_nr, $adres, $email_partner);

<form action="somefile.php" method="post">
<input type="text" name="firstname" id="firstname">
<input type="text" name="lastname" id="lastname">
<input type="number" name="age" id="age">
<input type="submit" name="submit" id="submit" value="Submit">
</form>

<?
if (isset($_POST['submit'])){
    $first = $_POST['firstname']; $last = $_POST['lastname'];
    $age = $_POST['age'];
    $body = "First name: ".$first." Last name: ".$last." Age: ".$age;
    mail("example@example.com", "sample subject", $body);
}
?>