Manual:$wgPHPSessionHandling

Details
Whether to use PHP session handling ($_SESSION and session_* functions) in SessionManager.

Possible values are:
 * : Integrate with PHP's session handling as much as possible.
 * : Integrate but log warnings if anything changes $_SESSION.
 * : Throw exceptions if PHP session handling is used.

If  is set to an invalid value (e.g. any string not listed above), it will automatically be set to warn.

If the constant  is defined,   is forced to 'disable'.

If the constant  is defined,   is ignored and PHP session handling will function independently of SessionHandler. SessionHandler and PHP's session handling may attempt to override each others' cookies.