To return the unique elements (those with a key that exists only once in either array but not in both) try:
function array_unique_diff ($array1, $array2)
{
  array_merge(array_diff_key($array1, $array2), array_diff_key($array2, $array1));
}

Example:
$array1 = array('blue'  => 1, 'red'  => 2, 'green'  => 3, 'purple' => 4);
$array2 = array('green' => 5, 'blue' => 6, 'yellow' => 7, 'cyan'   => 8);

  array_diff_key($array1, $array2)

returns

  array ( 'red' => 2, 'purple' => 4 ) 

  array_diff_key($array2, $array1)

returns

  array ( 'yellow' => 7, 'cyan' => 8, )

  array_unique_diff($array1, $array2);
  
returns

  array ( 'red' => 2, 'purple' => 4, 'yellow' => 7, 'cyan' => 8, )

Improved recursive version.

<?php
/**
 * @author Gajus Kuizinas <[email protected]>
 * @version 1.0.0 (2013 03 19)
 */
function array_diff_key_recursive (array $arr1, array $arr2) {
    $diff = array_diff_key($arr1, $arr2);
    $intersect = array_intersect_key($arr1, $arr2);
    
    foreach ($intersect as $k => $v) {
        if (is_array($arr1[$k]) && is_array($arr2[$k])) {
            $d = array_diff_key_recursive($arr1[$k], $arr2[$k]);
            
            if ($d) {
                $diff[$k] = $d;
            }
        }
    }
    
    return $diff;
}
?>

An up to date version is maintained at https://github.com/gajus/flow/blob/master/flow.inc.php#L337.

I needed something a little different where maybe even the keys in multidimensional arrays don't match up. Setting $assoc to false will cause only to check for missing keys, otherwise it compares values as well. This was also based on '2ge at 2ge dot us' function

<?php

function n_array_diff_assoc ($a1, $a2, $assoc=true) {
    $r = array();
    if(is_array(current($a1))):
        foreach($a1 as $k => $v):
            if(isset($a2[$k])):
                $diff = n_array_diff($a1[$k], $a2[$k], $assoc);
                if (!empty($diff)):
                    $r[$k] = $diff;
                endif;
            else:
                $r[$k] = $v;
            endif;
        endforeach;
    else:
        $r = $assoc ? array_diff_assoc($a1, $a2) : array_diff_key($a1, $a2);
    endif;
    return $r;
}
?>

Seems to be a great function, especially for n-dimensions arrays. The only problem is that I cannot find it in php 5.0.3 and 5.0.4. Does it really exist ?! :(

[20:27:05][maxence@conurb] ~/test2/php-5.0.4$ grep PHP_FUNCTION * -r | grep -i array_diff_key 
[20:27:09][maxence@conurb] ~/test2/php-5.0.4$

The recursive function suggested by '2ge at 2ge dot us' will provide you with empty arrays if there's no diff. 
This variant of the function cleans up empty arrays and fixes a bug in the first suggested version. It works 100%
.
<?php
function array_diff_key_recursive ($a1, $a2) {
        foreach($a1 as $k => $v) {
            //$r[$k] = is_array($v) ? $this->array_diff_key_recursive($a1[$k], $a2[$k]) : array_diff_key($a1, $a2);
        if (is_array($v))
            {
            $r[$k]=$this->array_diff_key_recursive($a1[$k], $a2[$k]);
            }else
            {
            $r=array_diff_key($a1, $a2);
            }

        if (is_array($r[$k]) && count($r[$k])==0)
            {
            unset($r[$k]);
            }
        }
        return $r;
    }
?>

The PHP4 version below works only unidirectionally. If you switch the arrays around i.e. (ar2, ar1) you get different results than (ar1, ar2).

Here's a simple function that returns true if all keys in the first array are found in the second array, and false if they aren't.

function same_keys ($a1, $a2) {
    $same = false;
    if (!array_diff_key($a1, $a2)) {
        $same = true;
        foreach ($a1 as $k => $v) {
            if (is_array($v) && !same_keys($v, $a2[$k])) {
                $same = false;
                break;
            }
        }
    }
    return $same;
}

To check if two arrays have the same structure, ignoring values, execute the function twice, the second time with the arguments reversed.

Well, you could implement in the code something more powerfull: 

http://www.php.net/manual/en/function.array-diff.php#31364

you can use this function for return the difference of two array !

<?php
function array_unique_diff_key ($array1, $array2)
{
  if (is_array($array1) && is_array($array2))
    return array_diff_key($array1, $array2) + array_diff_key($array2, $array1);
  else if (is_array($array1)) return $array1;
  else if (is_array($array2)) return $array2;
  else return array();
}
?>

One more alternative variant :)
<?
if (!function_exists('array_diff_key')) {
    function array_diff_key() {
        $argCount   = func_num_args();
        $diff_arg_prefix = 'diffArg';
        $diff_arg_names = array();
        for ($i=0; $i < $argCount; $i++) {
            $diff_arg_names[$i] = 'diffArg'.$i;
            $$diff_arg_names[$i] = array_keys((array)func_get_arg($i));
        }
        $diffArrString = '';
        if (!empty($diff_arg_names)) $diffArrString =  '$'.implode(', $', $diff_arg_names);
        eval("\$result = array_diff(".$diffArrString.");");
        return $result;
    }
}
?>