Programming PHP email form

Question

I ran into a problem with setting up my email form. My form sends to my designated email, but doesn't send the subject, or the sender's email. I'll paste my code.

<?php
$nameErr = $emailErr = $genderErr = $websiteErr = "";
$name = $email = $gender = $comment = $website = "";

if ($_SERVER["REQUEST_METHOD"] == "POST") {
    if (empty($_POST["name"])) {
        $nameErr = "Name is required";
    } else {
        $name = test_input($_POST["name"]);

        if (!preg_match("/^[a-zA-Z ]*$/",$name)) {
            $nameErr = "Only letters and white space allowed";
        }
    }

    if (empty($_POST["email"])) {
        $emailErr = "Valid email is required";
    } else {
        $email = test_input($_POST["email"]);

        if (!filter_var($email, FILTER_VALIDATE_EMAIL)) {
            $emailErr = "Invalid email format";
        }
    }

    if (empty($_POST["subject"])) {
        $comment = "";
    } else {
        $comment = test_input($_POST["comment"]);
    }

    function test_input($data) {
        $data = trim($data);
        $data = stripslashes($data);
        $data = htmlspecialchars($data);
        return $data;
    }
    header('Location: contact_thank_you.html');
    exit();
    ?>

Thank you in advance ! I simply cannot figure this one out.