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.