In your index.php file, you will need to load content based on the variable $_GET['page'].
When someone visits index.php?page=pbsetup.php, the value of $_GET['page'] will be 'pbsetup.php'. You can then include that page in your content.
NOTE: You should have a list of approved pages that can be loaded through this method to avoid security exploits. If you simply load whatever is passed in the URL, you will be hacked quickly. If you don't understand, you should get more familiar with PHP first before using this.