that just php's include path.
when you include a file php will first look for the file you specified relative to the current working directory. if it doesnt find the file, it will then look for that file realtive to its include path. if still not found, it will give you that error.
it just sounds like your file is in a diff directory than you think it is.
try this
$file = '../DB/DBConnect.php';
echo getcwd() . $file;
require($file);
that will output a full file path, and im sure you will see your problem right away.
btw- you may find it usefull to stop doing includes relative to your current working directory. you can also do like this
require($_SERVER['DOCUMENT_ROOT'] . '/path/to/script.php');