My attempt to recursively sort data in nested arrays and objects. This is helpful for sorting data in JSON by key.

JsonSortTest.php

<?php

class JsonSortTest extends PHPUnit_Framework_TestCase {
    public $fixtureFilename = 'fixture.json';
    public $outputFilename = 'output.json';
    public $fixtureJson;

    function ksortObjectsRecursive(&$data, $sortFlags = SORT_REGULAR) {
        if (!function_exists('ksortObjectsRecursiveCallback')) {
            function ksortObjectsRecursiveCallback(&$data, $unusedKey, $sortFlags) {
                $dataWasCastAsArray = false;
                if (is_object($data)) {
                    $data = (array)$data;
                    $dataWasCastAsArray = true;
                }

                $success = is_array($data) &&
                    ksort($data, $sortFlags) &&
                    array_walk($data, __FUNCTION__, $sortFlags);

                if ($dataWasCastAsArray) {
                    $object = new stdClass();
                    foreach ($data as $key => $value) {
                        $object->$key = $value;
                    }
                    $data = $object;
                }
                return $success;
            }
        }

        return ksortObjectsRecursiveCallback($data, null, $sortFlags);
    }

    function testJsonSorting() {
        $this->fixtureJson = file_get_contents($this->fixtureFilename);

        $jsonObjects = json_decode($this->fixtureJson);

        $this->ksortObjectsRecursive($jsonObjects);

        $jsonString = json_encode($jsonObjects, JSON_PRETTY_PRINT | JSON_UNESCAPED_SLASHES);
        file_put_contents($this->outputFilename, $jsonString);

        $this->assertJsonStringEqualsJsonFile($this->fixtureFilename, $jsonString);
    }
}

Admittedly, this has some limitations. When an object of any type is passed to the method, it will be returned as StdClass, rather than its original type. Also, this may run into access problems when trying to read or write certain object properties.