Thank you for this bug report. To properly diagnose the problem, we
need a short but complete example script to be able to reproduce
this bug ourselves. 

A proper reproducing script starts with <?php and ends with ?>,
is max. 10-20 lines long and does not require any external 
resources such as databases, etc. If the script requires a 
database to demonstrate the issue, please make sure it creates 
all necessary tables, stored procedures etc.

Please avoid embedding huge scripts into the report.

There is a testcase in a github repo referenced in this bug.

yeah, I've seen it, and it depends on atoum, and it makes the work for us easier if we don't have to get ourself familiar with each and every php project to be able to find out what exactly causes the problem,
I've went ahead and created a simple reproduce script based on your repo:

<?php
$sentence = 'Le système le style du couteau du suisse';
$words = explode(' ', $sentence);
$words = array_filter($words); // filter out extra spaces
$words = array_unique($words); // remove duplicate words
// sort words from longest to shortest
usort(
        $words,
        function ($a, $b) {
                return mb_strlen($b) - mb_strlen($a);
        }
);

var_dump($words);

the result of the sort is different, because mb_strlen returns a different value on 5.6, because the default mb_internal_encoding was changed in 5.6 from ISO-8859-1 to UTF-8:
http://3v4l.org/rZNL4
this change is described in our UPGRADING guide and should be explained in the php.net upgrading docs also:
http://git.php.net/?p=php-src.git;a=blob;f=UPGRADING;h=7a174c47c2a034ac3718cf364cde66569d72b0d7;hb=refs/heads/PHP-5.6#l80
if you want to make sure that your code/test produces the same result on both 5.6 and older versions (or anywhere where the internal encoding is set to a non-default value), consider explicitly passing the second optional $encoding argument to mb_strlen()

Thanks for investigating, I see that the bug was in my code as I do have mb_internal_encoding('UTF-8') in the bootstrap file for the application but I don't have it set in the bootstrap file used in my unit tests.