Detect if it's a singe article outside wordpress

Hello,
I am building a mobile version of the main website of my company.
Basically, I have created only an index.php page and include wp-load.php

I have written some custom function, so I can have the slug of each page/article (passing through a get variable).

Anyway, I would like to create a custom page for single articles. I’m trying the is_single function, but it doesn’t work.

Here is the code:

<?php $page = $_GET['p']; ?>

<?php // Special content for Single Article page ?>
<?php if( is_single( $page ) ) { ?>
<p>custom text here</p>
<?php } ?>

Any idea?

Thanks