Given a file test.ini like
  foo = BAR

and parsing it in parse.php:
  define ( 'BAR', 'just any value' );
  $ini = parse_ini_file ( 'test.ini' );
  print_r ( $ini );

will result in
  Array
  (
      [foo] => just any value
  )

This is maybe a quite useful behaviour but should at least be mentioned in the documentation. 
I would prefer a third argument added to the function which could be used to turn this feature off.