Most php files within Joomla! 1.0.x begin with the following statement:
defined( '_VALID_MOS' ) 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.
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