Array
(
[0] => Array
(
[file] => /var/websites/example.com/assets/images/200px/1419050406e6648e1c766551a0ffc91380fd6ff3406002011-10-233750.
As array_unique operates on flat arrays, you can not use it directly. But you can first map all 'md5'
values to a flat array, unique it and then get the elements with array_intersect_key:
$allMd5s = array_map(function($v) {return $v['md5'];}, $array);
$uniqueMd5s = array_unique($md5);
$result = array_intersect_key($arr, $uniqueMd5s);