Line 56 in the default installation reads
PHP Code:
  define('HEADING_TITLE', 'Congratulations! You have successfully installed your Zen Cart™ E-Commerce Solution.'); 
Note the two parameters between the brackets, enclosed by single apostrophes and separated by a comma.

Your equivalent line reads
define('');
and therefore has just one NULL parameter.

At the very least you should make this read
PHP Code:
  define('HEADING_TITLE', ''); 
(that's two single apostrophes at the end, rather than one double)