Individually sorting subarrays of an array in PHP

<?php

// http://stackoverflow.com/questions/26713294/individually-sorting-subarrays-of-an-array-in-php
// $arr1 gives the expected result, $arr2 does not,
// although array_walk() should do exactly the same as the foreach - apply sort() on all subarrays of $arr

$arr = array(
    array( 1, 1, 1, 2 ),
    array( 1, 2, array(1,1) ),
    array( 1, 2, array(1,1) ),
    array( 1, 2, array(1,1) ),
    array( 2, array(1,1,1) ),
    array( 1, 1, array(1,2) ),
    array( array(1,2), array(1,1) ),
    array( 1, 1, array(1,2) ),
    array( array(1,1), array(1,2) ),
    array( 1, array(1,1,2) ),
    array( 1, 1, array(1,2) ),
    array( array(1,1), array(1,2) ),
    array( 1, array(1,1,2) ),
    array( 1, array(1,1,2) ),
    array( array(1,1,1,2) )
);
// 15 rows

$arr1 = $arr;
foreach ($arr1 as $key => $val) {
    $derp = $val;
    sort($derp);
    $arr1[$key] = $derp;
}
sort($arr1);
$arr1 = array_map("unserialize", array_unique(array_map("serialize", $arr1)));

$arr2 = $arr;
array_walk($arr2, 'sort');
sort($arr2);
$arr2 = array_map("unserialize", array_unique(array_map("serialize", $arr2)));

echo array_values($arr1) == array_values(array(
    array( array(1,1,1,2) ),
    array( 1, array(1,1,2) ),
    array( 2, array(1,1,1) ),
    array( array(1,1), array(1,2) ),
    array( 1, 1, array(1,2) ),
    array( 1, 2, array(1,1) ),
    array( 1, 1, 1, 2 )
));
// 7 rows

echo array_values($arr2) == array_values(array(
    array( array(1,1,1,2) ),
    array( 1, array(1,1,2) ),
    array( 2, array(1,1,1) ),
    array( array(1,1), array(1,2) ),
    array( array(1,1,2), 1 ),
    array( 1, 1, array(1,2) ),
    array( 1, 2, array(1,1) ),
    array( 1, array(1,1), 2 ),
    array( 1, 1, 1, 2 )
));
// 9 rows
// array( 1, array(1,1), 2 )  is a duplicate of  array( 1, 2, array(1,1) )
// array( array(1,1,2), 1 )  is a duplicate of  array( 1, array(1,1,2) )

// this script will display two ones for the equalities
// and six times "Notice: Array to string conversion in ..." for the line that contains array_walk