Shell_exec不执行php文件

I'm trying to run a shell_exec query by pressing a button. But it does not seem to execute anything. I checked php.ini and it isn't disabled. I'm trying to execute a php file in a different directory
/var/www/html/root/download_handler.php. It doesn't seem to work.

Here's my code. I hope anyone knows what i can do to fix this?

if ($_SERVER['REQUEST_METHOD'] === "POST") {

if (isset($_POST['download'])) {

    $site = $_POST['site'];
    $wait = $_POST['wait'];
    $folder = $_POST['folder'];
    $strpos = strpos($site, "http://");

    if (empty($site) || empty($wait) || empty($folder)) {

        $error = Error::display("Input fields left empty", "alert");

    } else {

        if ($strpos === false) {

            $error = Error::display("Incorrect URL Format. Example: http://example.com/", "alert");

        } else {

            $username = $_SESSION['username'];


            $error = Error::display($site . $wait . $folder . $username, "success");
            shell_exec("screen php /var/www/html/download_handler.php download $site $folder $wait $username");


        }

    }

}

}

?>