如何有条件地在PHP中包含一个文件

I have made a website and it works fine.. But now I want to be able to have non computer minded people be able to change if one line shows up or not.. I was going to achive this by having a line of php include

<?php include 'on.php'; ?>

and they name the file "on" to have it display the text in the php file this works fine.. But then when the name it off.. it displays a error (obviously because it cant find anything) but i would like to keep my set up the same but remove the error meassage or maybe have a if statement saying that if it cant find anything do nothing.

How would/should i go about this?

<?php
$filename = 'on.php'; // or whatever

if (is_readable($filename)) {
    include $filename;
}