Useful PHP script when you want to merge multiple arrays like Y.merge().

array_merge_recursive_distinct.php

<?php
/**
 * Copy from http://www.php.net/manual/en/function.array-merge-recursive.php#92195 
 * 
 * array_merge_recursive does indeed merge arrays, but it converts values with duplicate
 * keys to arrays rather than overwriting the value in the first array with the duplicate
 * value in the second array, as array_merge does. I.e., with array_merge_recursive,
 * this happens (documented behavior):
 *
 * array_merge_recursive(array('key' => 'org value'), array('key' => 'new value'));
 *     => array('key' => array('org value', 'new value'));
 *
 * array_merge_recursive_distinct does not change the datatypes of the values in the arrays.
 * Matching keys' values in the second array overwrite those in the first array, as is the
 * case with array_merge, i.e.:
 *
 * array_merge_recursive_distinct(array('key' => 'org value'), array('key' => 'new value'));
 *     => array('key' => array('new value'));
 *
 * Parameters are passed by reference, though only for performance reasons. They're not
 * altered by this function.
 *
 * @param array $array1
 * @param array $array2
 * @return array
 * @author Daniel <daniel (at) danielsmedegaardbuus (dot) dk>
 * @author Gabriel Sobrinho <gabriel (dot) sobrinho (at) gmail (dot) com>
 */
function array_merge_recursive_distinct(array &$array1, array &$array2)
{
    $merged = $array1;
    foreach ($array2 as $key => &$value)
    {
        if (is_array($value) && isset($merged[$key]) && is_array($merged[$key]))
        {
            $merged[$key] = array_merge_recursive_distinct($merged[$key], $value);
        }
        else
        {
            $merged[$key] = $value;
        }
    }
    return $merged;
}

?>

test.php

<?php
function array_merge_recursive_distinct(array &$array1, array &$array2)
{
    $merged = $array1;
    foreach ($array2 as $key => &$value)
    {
        if (is_array($value) && isset($merged[$key]) && is_array($merged[$key]))
        {
            $merged[$key] = array_merge_recursive_distinct($merged[$key], $value);
        }
        else
        {
            $merged[$key] = $value;
        }
    }
    return $merged;
}

$config = array(

    /**
     * CSS/JS seed URLs.
     */
    "seed" => array(
        "css" => "combo/?g=css",
        "js"  => "combo/?g=js",
    ),

    /**
     * Combo base URL.
     */
    "base" => "combo/?f=",

    /**
     * Combo separator.
     */
    "separator" => ",",

    /**
     * YUI default configuration.
     */
    "yui" => array(
        "config" => array(
            "lang" => "en-US",
        ),
        "callback" => "(new Y.ModuleManager()).startAll();",
    ),
);

$options = array(
 "yui" => array(
     "config" => array(
         "lang" => "zh-TW",
     ),
 ),
);

print_r(array_merge_recursive_distinct($config, $options));

?>

Thanks a lot!
I added an option to only replace truthy values (useful when you don't want empty strings to overwrite existing strings):

function array_merge_recursive_distinct ($array1, $array2, $only_reaplce_truthy_values = false) {
  $merged = $array1;
  foreach ($array2 as $key => $value) {
    if (is_array($value) && isset($merged[$key]) && is_array($merged[$key])) {
      $merged[$key] = array_merge_recursive_distinct($merged[$key], $value, $only_reaplce_truthy_values);
    } else {
      if ($only_reaplce_truthy_values) {
        $merged[$key] = $value ?: $array1[$key];
      } else {
        $merged[$key] = $value;
      }
    }
  }
  return $merged;
}