This is because natsort and natcasesort maintain the key association (it does not re-key the array).

An array must have sequential keys in order for json_encode to encode it as a natural array, otherwise it converts the array to an associative array (an object) and maintains the key value association.

If you use array_values on the resulting array after sorting it; it'l resolve your issue (relevant reference: http://php.net/manual/en/function.natcasesort.php#32610)

Example:

$array = array('b', 'a', 'c');
natsort($array);
$array = array_values($array);
echo "After natsort:<br>\r\n", json_encode($array), "<br>\r\n";

$array = array('b', 'a', 'c');
natcasesort($array);
$array = array_values($array);
echo "After natcasesort:<br>\r\n", json_encode($array), "<br>\r\n";

Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

Hello again,

I've read the documentation before creating this bug report, but I still didn't get it after I found out how it behaves. The documentation simply doesn't sound like "be aware your numeric keys will turn into strings".

It says:
>> This function implements a sort algorithm that orders alphanumeric strings in the way a human being would while maintaining key/value associations. This is described as a "natural ordering". An example of the difference between this algorithm and the regular computer string sorting algorithms (used in sort()) can be seen in the example below. <<

The information must be hidden in "while maintaining key/value associations", but it is not clear which consequences this has.
An additional information like this would be fine:
>> The maintenance of the key/value associations is performed via conversion of the numeric keys into strings. Unlike sort() elements will be accessed after sorting via their old index (e. g. $array[0] will return the element with index 0 and not the first element). If you don't need this behaviour, call array_values after sort or use usort with 'strnatcasecmp'. <<

The same goes for natcasesort. It would be very fine when you add something like that to the documentation.

Kind Regards
David Gausmann