Description:
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The session.use_strict_mode runtime configuration option (PHP 5.5.2+) is now described in the manual like this:

session.use_strict_mode boolean
session.use_strict_mode specifies whether the module will use strict session id mode. If this mode is enabled, the module does not accept uninitialized session ID. If uninitialized session ID is sent from browser, new session ID is sent to browser. Applications are protected from session fixation via session adoption with strict mode. Defaults to 0 (disabled). 

However, this is not accurate. As noted in https://bugs.php.net/bug.php?id=66947, current behavior is related *only* to built-in session save handlers - file and mm. Once you use custom save handler, the setting is meaningless. 

As this is security setting related to CVE-2011-4718, this behavior should be documented. I propose something like this:
Note: session.use_strict_mode applies only to PHP's built-in session save handlers. If you use custom save handler, you must implement the strict mode yourselves.