PHP write POST to file

if (isset($_POST) && isset($_POST["text"]) && isset($_POST["filename"])) {
    $myfile = fopen($_POST["filename"].".txt", "w") or die("Unable to open file!");
    $txt = $_POST["text"];
    fwrite($myfile, $txt);
    fclose($myfile);
} else {
    die('either text or filename isn\'t set');
}