Use symfony memcached session handler in Contao

initialize.php

<?php

...

/**
 * Symfony session storage
 */
$memcached = new \Memcached();
$memcached->addServer($GLOBALS['TL_CONFIG']['sessionMemcachedHost'], $GLOBALS['TL_CONFIG']['sessionMemcachedPort']);

require TL_ROOT . '/composer/vendor/symfony/http-foundation/Symfony/Component/HttpFoundation/Session/Storage/Proxy/AbstractProxy.php';
require TL_ROOT . '/composer/vendor/symfony/http-foundation/Symfony/Component/HttpFoundation/Session/Storage/Proxy/SessionHandlerProxy.php';
require TL_ROOT . '/composer/vendor/symfony/http-foundation/Symfony/Component/HttpFoundation/Session/SessionBagInterface.php';
require TL_ROOT . '/composer/vendor/symfony/http-foundation/Symfony/Component/HttpFoundation/Session/Storage/MetadataBag.php';
require TL_ROOT . '/composer/vendor/symfony/http-foundation/Symfony/Component/HttpFoundation/Session/Storage/Handler/MemcachedSessionHandler.php';
require TL_ROOT . '/composer/vendor/symfony/http-foundation/Symfony/Component/HttpFoundation/Session/Storage/SessionStorageInterface.php';
require TL_ROOT . '/composer/vendor/symfony/http-foundation/Symfony/Component/HttpFoundation/Session/Storage/NativeSessionStorage.php';

register_shutdown_function(function() {
	\Session::getInstance()->__destruct();
});

new \Symfony\Component\HttpFoundation\Session\Storage\NativeSessionStorage(
	[],
	new \Symfony\Component\HttpFoundation\Session\Storage\Handler\MemcachedSessionHandler(
		$memcached,
		['prefix' => $GLOBALS['TL_CONFIG']['sessionMemcachedPrefix'] . $GLOBALS['TL_CONFIG']['encryptionKey'] . '_']
	)
);


/**
 * Start the session
 */
@session_set_cookie_params(0, (TL_PATH ?: '/')); // see #5339
@session_start();

...

localconfig.php

<?php

### INSTALL SCRIPT START ###
$GLOBALS['TL_CONFIG']['sessionMemcachedHost'] = 'localhost';
$GLOBALS['TL_CONFIG']['sessionMemcachedPort'] = 11211;
$GLOBALS['TL_CONFIG']['sessionMemcachedPrefix'] = 'contao_session_';
### INSTALL SCRIPT STOP ###

This core hacks requires to manipulate the system/initialize.php!
First install symfony/http-foundation with the composer client, then add the snippet above bevore the "Start the session" block.
Warning: The "Start the session" block from the snippet above already exist in the initialize.php! Do not copy it twice!

Now it is configurable, put these three options into your localconfig.php and update the init script. Then you can change the settings from your localconfig.php. This may be helpful, when using syncCto or something similar.