Description:
------------
when some elements in array are changed by reference and this array is then converted using mb_convert_encoding, these elements are removed or empty. Warning is emmited "mb_convert_encoding(): Object is not supported in ...", but function returns result without mentioned elements. Best see in code example.

Test script:
---------------
// any sample array, structure does not matter
$array = [
    'ads' => [
        123 => ['name' => 'this', 'id' => 444],
        234 => ['name' => 'that', 'id' => 555],
    ],
    'other' => ['one', 'two']
    ];
         
// we modify array elements using reference     
foreach( $array['ads'] as &$ad ){
    $ad['premium'] = (int)($ad['id'] == 555);
}

// print_r prints array contents OK
print_r($array);

// mb_convert_encoding emits Warning, but returns result where modified array elements are removed - in this case 'ads' element

print_r(mb_convert_encoding($array, 'UTF-8', 'UTF-8'));

Expected result:
----------------
array elements does not disappear, mb_convert_encoding does not emit warning

Actual result:
--------------
mb_convert_encoding returns result where modified array elements are removed. mb_convert_encoding emits warning