If you are storing your data via the WordPress "option" mechanism, then just add the following code to the top of your php file in your plugin folder to access the string you have saved in the database:
PHP Code:
<?php require('../../../wp-blog-header.php'); ?>
Don't forget to output the file as CSS, not text/HTML or the browser may not render it correctly. The following will output a CSS file which contains your blogs URL. Just replace the bloginfo with whatever code you want to display in the file.
PHP Code:
<?php
require('../../../wp-blog-header.php');
header('Content-type: text/css');
echo bloginfo('url');
?>