This is the below error which is getting logged in the error log file
PHP Warning: require_once() [<a href='function.require-once'>function.require-once</a>]: URL file-access is disabled in the server configuration in on line 10
PHP Warning: require_once(http://www.domain.com/paypal.inc.php) [<a href='function.require-once'>function.require-once</a>]: failed to open stream: no suitable wrapper could be found in ipn.php on line 10
PHP Fatal error: require_once() [<a href='function.require'>function.require</a>]: Failed opening required 'http://domain.com/paypal.inc.php' (include_path='.:/php') on line 10
If the file is on your own server, you shouldnt be trying to include/require it using an http:// reference. Including PHP files from external sites means that you include the -output- of the file only, not the source code.