The array_diff_assoc_array from "chinello at gmail dot com" (and others) will not work for arrays with null values. That's because !isset is true when an array key doesn't exists or is set to null.

(sorry for the changed indent-style)
<?php
function array_diff_assoc_recursive($array1, $array2) {
    $difference=array();
    foreach($array1 as $key => $value) {
        if( is_array($value) ) {
            if( !isset($array2[$key]) || !is_array($array2[$key]) ) {
                $difference[$key] = $value;
            } else {
                $new_diff = array_diff_assoc_recursive($value, $array2[$key]);
                if( !empty($new_diff) )
                    $difference[$key] = $new_diff;
            }
        } else if( !array_key_exists($key,$array2) || $array2[$key] !== $value ) {
            $difference[$key] = $value;
        }
    }
    return $difference;
}
?>

And here an example (note index 'b' in the output):
<?php
$a1=array( 'a' => 0, 'b' => null, 'c' => array( 'd' => null ) );
$a2=array( 'a' => 0, 'b' => null );

var_dump( array_diff_assoc_recursive( $a1, $a2 ) );
var_dump( chinello_array_diff_assoc_recursive( $a1, $a2 ) );
?>
array(1) {
  ["c"]=>
  array(1) {
    ["d"]=>
    NULL
  }
}

array(2) {
  ["b"]=>
  NULL
  ["c"]=>
  array(1) {
    ["d"]=>
    NULL
  }
}

The following will recursively do an array_diff_assoc, which will calculate differences on a multi-dimensional level.  This not display any notices if a key don't exist and if error_reporting is set to E_ALL:

<?php
function array_diff_assoc_recursive($array1, $array2)
{
    foreach($array1 as $key => $value)
    {
        if(is_array($value))
        {
              if(!isset($array2[$key]))
              {
                  $difference[$key] = $value;
              }
              elseif(!is_array($array2[$key]))
              {
                  $difference[$key] = $value;
              }
              else
              {
                  $new_diff = array_diff_assoc_recursive($value, $array2[$key]);
                  if($new_diff != FALSE)
                  {
                        $difference[$key] = $new_diff;
                  }
              }
          }
          elseif(!isset($array2[$key]) || $array2[$key] != $value)
          {
              $difference[$key] = $value;
          }
    }
    return !isset($difference) ? 0 : $difference;
}
?>

[NOTE BY danbrown AT php DOT net: This is a combination of efforts from previous notes deleted.  Contributors included (Michael Johnson), (jochem AT iamjochem DAWT com), (sc1n AT yahoo DOT com), and (anders DOT carlsson AT mds DOT mdh DOT se).]

The direction of the arguments does actually make a difference:

<?php
$a = array(
    'x' => 'x',
    'y' => 'y',
    'z' => 'z',
    't' => 't',
);

$b = array(
    'x' => 'x',
    'y' => 'y',
    'z' => 'z',
    't' => 't',
    'g' => 'g',
);

print_r(array_diff_assoc($a, $b));
print_r(array_diff_assoc($b, $a));
?>

echoes:

Array
(
)
Array
(
    [g] => g
)

Hi all,
For php versions < 4.3...

<?php
/**
* array_diff_assoc for version < 4.3
**/
if (!function_exists('array_diff_assoc'))
{
    function array_diff_assoc($a1, $a2)
    {
        foreach($a1 as $key => $value)
        {
            if(isset($a2[$key]))
            {
                if((string) $value !== (string) $a2[$key])
                {
                     $r[$key] = $value;
                }
            }else
            {
                $r[$key] = $value;
            }
        }
        return $r ;
    }
}

?>

To unset elements in an array if you know the keys but not the values, you can do:

<?php
$a = array("foo", "bar", "baz", "quux");
$b = array(1, 3); // Elements to get rid of

foreach($b as $e)
  unset($a[$e]);
?>

Of course this makes most sense if $b has many elements or is dynamically generated.

To diff between n-dimensional array, juste use this :

<?php
function array_diff_values($tab1, $tab2)
    {
    $result = array();
    foreach($tab1 as $values) if(! in_array($values, $tab2)) $result[] = $values;
    return $result;
    }
?>

NOTE: the diff_array also removes all the duplicate values that match to the values in the second array:

<?php
    $array1 = array("a","b","c","a","a");
    $array2 = array("a");

    $diff = array_diff($array1,$array2);

    // yields: array("b","c") the duplicate "a" values are removed
?>

an earlier post for recursive array_diff_assoc failed because isset returned false on an array element containing a null value. I updated the code so it compares null values too.

<?php
function array_diff_assoc_recursive($array1, $array2)
{
    foreach($array1 as $key => $value)
    {
        if(is_array($value))
        {
              if(!isset($array2[$key]))
              {
                  $difference[$key] = $value;
              }
              elseif(!is_array($array2[$key]))
              {
                  $difference[$key] = $value;
              }
              else
              {
                  $new_diff = array_diff_assoc_recursive($value, $array2[$key]);
                  if($new_diff != FALSE)
                  {
                        $difference[$key] = $new_diff;
                  }
              }
          }
          elseif(!array_key_exists($key, $array2) || $array2[$key] != $value)
          {
              $difference[$key] = $value;
          }
    }
    return !isset($difference) ? 0 : $difference;
}
?>