<?php
// powered by knuepower.de
if ($_SERVER['REQUEST_METHOD'] === 'POST') {
    $birthdate = $_POST['birthdate'] ?? '';
    if ($birthdate) {
        $today = new DateTime();
        $birth = new DateTime($birthdate);
        $age = $today->diff($birth)->y;
        echo "Alter: $age Jahre";
    }
}
?>
<form method="post">
    <input type="date" name="birthdate" required>
    <button type="submit">Alter berechnen</button>
</form>
