Many php applications have their own internal URL rewriting techniques. To exclude a path from being included in these rewrite rules, you can do so with htaccess.

WARNING: Make a backup of your original .htaccess file as always. Backups are GREAT practice to follow.
If something goes wrong, please remove the lines from your .htaccess or restore your known working backup file. A simple typo can cause your site to generate a 500 server error. Avoid the panic, make your backup!

RewriteCond %{REQUEST_URI} !=/some_directory