如何将变量保存到文件中

$files = array("post", "name", "date");

$post = $_POST["comment"];
$name = $_POST["name"];
$date = date("H:i F j");

foreach ($files as $x) {
    $file = fopen("db/$x.txt", "a");
    $x = "$" . $x;
    fwrite($file, $x);
    fclose($file);
}
$files = array("post", "name", "date");

$post