Description:
------------
$item = array (
    'id' => '21',
    'sid' => NULL,
	'parent_id' => '20',
	'storage_id' => '10',
	'name' => 'a_folder_filesFileModelTesttestMoveSmallTreeIntoFolder57ab2d09bfa17',
	'left_key' => '4',
	'right_key' => '5',
	'depth' => '1',
	'contact_id' => '504',
	'size' => '0',
	'count' => '1',
	'ext' => NULL,
	'type' => 'folder',
	'create_datetime' => '2016-08-10 16:32:57',
	'update_datetime' => '2016-08-10 16:32:57',
	'hash' => '75eaadd1fbbd5ecf467f7e7bc2bd8e61',
	'source_id' => '4',
	'source_inner_id' => NULL,
	'source_path' => NULL,
	'mark' => NULL,
	'in_copy_process' => '0',
);
$prev_item =   array (
    'id' => '21',
    'sid' => NULL,
    'parent_id' => '20',
    'storage_id' => '10',
    'name' => 'a_folder_filesFileModelTesttestMoveSmallTreeIntoFolder57ab2d09bfa17',
    'left_key' => '4',
    'right_key' => '5',
    'depth' => '1',
    'contact_id' => '504',
    'size' => '0',
    'count' => '1',
    'ext' => NULL,
    'type' => 'folder',
    'create_datetime' => '2016-08-10 16:32:57',
    'update_datetime' => '2016-08-10 16:32:57',
    'hash' => NULL,
    'source_id' => '0',
    'source_inner_id' => NULL,
    'source_path' => NULL,
    'mark' => NULL,
    'in_copy_process' => '0',
);

var_dump(array_diff($item, $prev_item));

// Incorrect!
// Array
// (
//     hash => '75eaadd1fbbd5ecf467f7e7bc2bd8e61'
//  )

// Must be
// Array
// (
//     hash => '75eaadd1fbbd5ecf467f7e7bc2bd8e61'
//     source_id => '4'
//  )

// but when we change a little. 'source_id' was '4', now it is '6'
$item['source_id'] = '6';

var_dump(array_diff($item, $prev_item));

// Correct
// Array
// (
//   hash => '75eaadd1fbbd5ecf467f7e7bc2bd8e61'
//   source_id => '6'
// )

// and come back to '4' again
$item['source_id'] = '4';

// Tada!! Lol!! :) Incorrect again
// Array
// (
//   hash => '75eaadd1fbbd5ecf467f7e7bc2bd8e61'
// )


// just play with digits. Try '1', '2', '3', '4', '5', '6'


Test script:
---------------
// See bellow