PHP: use array_unique on an array of arrays?

I have an array

Array(
[0] => Array
    (
        [0] => 33
        [user_id] => 33
        [1] => 3
        [frame_id] => 3
    )

[1] => Array
    (
        [0] => 33
        [user_id] => 33
        [1] => 3
        [frame_id] => 3
    )

[2] => Array
    (
        [0] => 33
        [user_id] => 33
        [1] => 8
        [frame_id] => 8
    )

[3] => Array
    (
        [0] => 33
        [user_id] => 33
        [1] => 3
        [frame_id] => 3
    )

[4] => Array
    (
        [0] => 33
        [user_id] => 33
        [1] => 3
        [frame_id] => 3
    )

)

As you can see key 0 is the same as 1, 3 and 4. And key 2 is different from them all.

When running the array_unique function on them, the only left is

Array (
[0] => Array
    (
        [0] => 33
        [user_id] => 33
        [1] => 3
        [frame_id] => 3
    )
)

Any ideas why array_unique isn't working as expected?

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.