Most PHP files within Joomla! begin with the following statement:
// no direct access defined( '_JEXEC' ) or die( 'Restricted access' );
This statement checks to see if the file is being called from within a Joomla! session. This protects your site by making it more difficult for a cracker/hacker to damage your site.
However, this line should NOT be included in your main index.php file, since this is the program that starts the Joomla! session.
Code for 1.0 extensions
// no direct access defined( '_VALID_MOS' ) or die( 'Restricted access' );