Manual:$wgPHPSessionHandling

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

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

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

If the constant MW_NO_SESSION is defined, $wgPHPSessionHandling is forced to 'disable'.

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