How do I use array_unique on an array of arrays?

It's because array_unique compares items using a string comparison. From the docs:

Note: Two elements are considered equal if and only if (string) $elem1 === (string) $elem2. In words: when the string representation is the same. The first element will be used.

The string representation of an array is simply the word Array, no matter what its contents are.

You can do what you want to do by using the following:

$arr = array(
    array('user_id' => 33, 'frame_id' => 3),
    array('user_id' => 33, 'frame_id' => 3),
    array('user_id' => 33, 'frame_id' => 8)
);

$arr = array_intersect_key($arr, array_unique(array_map('serialize', $arr)));

//result:
array
  0 => 
    array
      'user_id' => int 33
      'user' => int 3
  2 => 
    array
      'user_id' => int 33
      'user' => int 8

Here's how it works:

1. Each array item is serialized. This will be unique based on the array's contents.

2. The results of this are run through array_unique, so only arrays with unique signatures are left.

3. array_intersect_key will take the keys of the unique items from the map/unique function (since the source array's keys are preserved) and pull them out of your original source array.

Here's an improved version of @ryeguy's answer:

<?php

$arr = array(
    array('user_id' => 33, 'tmp_id' => 3),
    array('user_id' => 33, 'tmp_id' => 4),
    array('user_id' => 33, 'tmp_id' => 5)
);


# $arr = array_intersect_key($arr, array_unique(array_map('serialize', $arr)));
$arr = array_intersect_key($arr, array_unique(array_map(function ($el) {
    return $el['user_id'];
}, $arr)));

//result:
array
  0 => 
    array
      'user_id' => int 33
      'tmp_id' => int 3

First, it doesn't do unneeded serialization. Second, sometimes attributes may be different even so id is the same.

The trick here is that array_unique() preserves the keys:

$ php -r 'var_dump(array_unique([1, 2, 2, 3]));'
array(3) {
  [0]=>
  int(1)
  [1]=>
  int(2)
  [3]=>
  int(3)
}

This let's array_intersect_key() leave the desired elements.

I've run into it with Google Places API. I was combining results of several requests with different type of objects (think tags). But I got duplicates, since an object may be put into several categories (types). And the method with serialize didn't work, since the attrs were different, namely, photo_reference and reference. Probably these are like temporary ids.

array_unique() only supports multi-dimensional arrays in PHP 5.2.9 and higher.

Instead, you can create a hash of the array and check it for unique-ness.

$hashes = array(); 

foreach($array as $val) { 
    $hashes[md5(serialize($val))] = $val; 
} 

array_unique($hashes);