PHP 7 did change how sorting sometimes works, but it has never promised anything more than a sorted result - so long as the comparison method used is reliable. That means exactly one of $a<$b, $a==$b, and $a>b must always be true for each pair of array elements $a,$b that can be compared. If the comparison method is not reliable then the result is undefined.

PHP 7 also uses a more stable sort, meaning that two equal items will probably appear in the same order in the output as they did in the input. PHP 5 didn't do that and they would often be shuffled around.

The bug here is that your array is not sortable using the "normal" comparison method because those two subarrays are not comparable.
  https://3v4l.org/G1J0B
See how all three of the tests show false? That means you'll get an undefined sort order in the final "sorted" array. It only changed with PHP 7 by sheer luck.

To properly sort that array you need to use usort() with a custom comparison function <https://3v4l.org/HZXON> or to use array_multisort() with a separate array that is sortable <https://3v4l.org/BarYS>.