erro linha 17 19 21

<html>
<head>
    <title></title>
</head>
<body>

    <form action="formulario.php" method="post">
        <label>Nome:</label>
        <input type="text" name="nome" value="">
        <label>E-mail:</label>
        <input type="text" name="email" value="">
        <label>Mensagem</label>
        <textarea name="mensagem"></textarea>
        <input type="submit" value="submit">
    </form>
<?php
    echo $_POST['nome'];
    echo "<br>";
    echo $_POST['email'];
    echo "<br>";
    echo $_POST['mensagem'];
?>

</body>
</html>