Memcached – Migrating website from php 5.2 to php 5.6, encountering memcache errors

EDIT:
perhaps I could make this simpler: when my website is running on php-5.2, everything works. When i try to run it on php-5.6 I get this error:

The installed libraries are all the same:

RUN 
  apt-get install -y 
    php5-memcache 
    memcached 
    libapache2-mod-php5

any ideas?

Note* I have tried installing php5-memcached, and this is what happens next.

After installing php5-memcache,

I get this warning message:

Message: Memcached::__construct() expects parameter 1 to be string, array given

Then the page break with a Fatal Error:

Fatal error: Call to a member function get() on null in  auth_model.php on line 800 E_ERROR Error in file  auth_model.php

This may be the breaking code ( auth_model.php on line 800):

    $memcached_profile=$this->memcached->get($memcached_key);

But it seems more like its memcache that’s not working

These are the versions:
Current PHP version: 5.6.40-0+deb8u4
Current CodeIgniter version: 2.0.2
Memcache version: 1.4.21

>php -i | grep memcache  
libmemcached version => 1.0.18

by the way, it makes no difference whether the memcached service is started or not!