require_once :failed to open stream: no such file or directory
The error pretty much explains what the problem is: you are trying to include a file that is not there.
Try to use the full path to the file, using realpath(), and use dirname(__FILE__) to get your current directory:
require_once(realpath(dirname(__FILE__) . '/../includes/dbconn.inc'));
You will need to link to the file relative to the file that includes eventManager.php (Page A)
Change your code fromrequire_once('../includes/dbconn.inc');
Torequire_once('../mysite/php/includes/dbconn.inc');
this will work as well
require_once(realpath($_SERVER["DOCUMENT_ROOT"]) .'/mysite/php/includes/dbconn.inc');