Как исправить ошибку PHP Warning: Cannot modify header information – headers already sent by

<form action="pay.php" method="POST">
    <input type="radio" value="https://vk.com/feed">
    <input type="radio" value="https://twitch.tv">
    <input type="submit">
</form>

<?php
if (isset($_POST['value'])) {$value = $_POST['value'];}
header('Location: ' . $value);
?>

Источник: https://ru.stackoverflow.com/questions/1210282/%D0%9A%D0%B0%D0%BA-%D0%B8%D1%81%D0%BF%D1%80%D0%B0%D0%B2%D0%B8%D1%82%D1%8C-%D0%BE%D1%88%D0%B8%D0%B1%D0%BA%D1%83-php-warning-cannot-modify-header-information-headers-alr