php developer blog

Using array_unique() with multidimensional arrays

by Dominik Jungowski

31. Januar 2009 comments feed

There's one problem with array_unique(): It doesn't work with multidimensional arrays. Here's an example:

$array = array(
    array(
        'id'    => 123,
        'name'  => 'Some Product',
        'ean'   => '1234567890123'
    ),
    array(
        'id'    => 123,
        'name'  => 'Some Product',
        'ean'   => '4852950174938'
    ),
    array(
        'id'    => 123,
        'name'  => 'Some Product',
        'ean'   => '1234567890123'
    ),
);
$uniqueArray = array_unique($array);
var_dump($uniqueArray);

Two elements are exactly the same, but one element has a different EAN, yet the var_dump() returns the following:

array(1) {
  [0]=>
  array(3) {
    ["id"]=>
    int(123)
    ["name"]=>
    string(12) "Some Product"
    ["ean"]=>
    string(13) "1234567890123"
  }
}

Obviously this is unexpected behaviour. array_unique() threw out the second element, which is clearly not the same as Element 1 and 3. The easiest way I came across is using md5 hashes for comparison of the elements. All you need is to iterate over the first dimension, serialize it and create a MD5 hash of it for comparison:

/**
 * Create Unique Arrays using an md5 hash
 *
 * @param array $array
 * @return array
 */
function arrayUnique($array, $preserveKeys = false)
{
    // Unique Array for return
    $arrayRewrite = array();
    // Array with the md5 hashes
    $arrayHashes = array();
    foreach($array as $key => $item) {
        // Serialize the current element and create a md5 hash
        $hash = md5(serialize($item));
        // If the md5 didn't come up yet, add the element to
        // to arrayRewrite, otherwise drop it
        if (!isset($arrayHashes[$hash])) {
            // Save the current element hash
            $arrayHashes[$hash] = $hash;
            // Add element to the unique Array
            if ($preserveKeys) {
                $arrayRewrite[$key] = $item;
            } else {
                $arrayRewrite[] = $item;
            }
        }
    }
    return $arrayRewrite;
}

$uniqueArray = arrayUnique($array);
var_dump($uniqueArray);

Now the result is the one array_unique() should have already given:

array(2) {
  [0]=>
  array(3) {
    ["id"]=>
    int(123)
    ["name"]=>
    string(12) "Some Product"
    ["ean"]=>
    string(13) "1234567890123"
  }
  [1]=>
  array(3) {
    ["id"]=>
    int(123)
    ["name"]=>
    string(12) "Some Product"
    ["ean"]=>
    string(13) "4852950174938"
  }
}

This works with as many dimensions as you like.