Manual:$wgSessionInsecureSecrets/ja

詳細
If for some reason you can't install the PHP OpenSSL or mcrypt extensions, you can set this to true to make MediaWiki work again at the cost of storing sensitive session data insecurely.

But it would be much more secure to just install the OpenSSL extension.

$wgSessionInsecureSecrets を有効にすることはお勧め できません !