Manual:$wgSessionInsecureSecrets/en

Details
MediaWiki will not work out of the box if the PHP OpenSSL or mcrypt extensions are not available on the server. If for some reason you cannot install one of these extensions, you can set $wgSessionInsecureSecrets to "true" to make MediaWiki work again. This comes at the cost of storing sensitive session data insecurely.

If one of the two extensions is installed, $wgSessionInsecureSecrets is not set, but none of the cipher algorithms, which MediaWiki supports, is available in that extension, you will get this error message:

Warning: openssl_encrypt: Unknown cipher algorithm in /wiki/includes/session/Session.php on line 424

This can be fixed like so: In order for OpenSSL to be able to work correctly, you will need one of these cipher algorithm: aes-256-ctr or aes-256-cbc. If you are using mcrypt, it needs to support the rijndael-128 algorithm and either ctr or cbc mode.

Enabling  is NOT recommended! Note that it is the best solution to install the OpenSSL extension.