I saw the same in the PEAR core unit tests.

This only happens when the elements are equal. Since they are equal, the order of the elements is unspecified and dependent on the internal implementation and the sort algorithm.

Not a bug in my eyes.

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.

Sounds like cweiske's right. PHP's sorting has never promised that equal elements will appear in any particular order. Also know that PHP 7 uses a different sorting algorithm for small arrays than it does larger arrays.

This report seems related: https://bugs.php.net/bug.php?id=40888

If it's not a bug and indeed related to small arrays in php7, I would rather see a small notice in the upgrade guide than to have people "waste" time on such a trivial "issue" as it's much better to rely on defined behavior by the developer.

I agree that a note in UPGRADING is appropriate, event though
the documentation (e.g. for usort()) already states:

| If two members compare as equal, their relative order in the
| sorted array is undefined.

See also the related internal discussion
<http://markmail.org/thread/trb3g4o6vqccitpd>.

Here is live example with this bug: https://3v4l.org/AErRF
Neither values nor keys are the same.
Here is example:
<?php
$array = array(
    'is' => 'a',
    'b'
);

ksort($array);
var_dump($array);

nazar, actually those are the same. You are comparing integer 0 to string "is" there which means PHP will cast "is" to int which is 0. 0==0.
You can prove this to yourself by forcing a string sort:

    ksort($array, SORT_STRING);