PHPDoc (of the public API) says that this method returns string but it also returns null, which is not allowed in some method calls. This fixes that behaviour and returns an empty string and fixes all code paths that explicitly checked for null to be still compliant.

Found while enabling the strict_typing for lib/private for the PHP7+ migration.

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

* Return types
* Typehints
* made strict
* fix tests

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

Apparently scan leads to some issues sometimes on cluster. So just use
the get function to fetch the keys.

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

Signed-off-by: Morris Jobke <hey@morrisjobke.de>

Signed-off-by: Joas Schilling <coding@schilljs.com>

Signed-off-by: Joas Schilling <coding@schilljs.com>

Signed-off-by: Roeland Jago Douma <roeland@famdouma.nl>

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>

Signed-off-by: Lukas Reschke <lukas@statuscode.ch>

Fix an issue with APCus inc and dec methods on PHP 7

see https://github.com/krakjoe/apcu/issues/183#issuecomment-244038221 for details

* allow configuration of memcache options

Use production values for memcached as explained in http://apprize.info/php/scaling/15.html

The current implementiation uses ascii based serialization. This PR should reduce traffic to the memcached server.

cc @MorrisJobke @FelixBoehm

* add config sample

* merge config options, throw hint on config error

* fix typo

* fix config sample