The goal is to find a nice way to merge multiple configs where the inner-configs ($localConfig) are able to unset keys of the outer-configs ($globalConfig)

gistfile1.php

<?php

// Global Configuration
$globalConfig = array
(
  'key1' => 'val1',
  'key2' => array
  (
    'key2.1' => 'val2.1',
    'key2.2' => 'val2.2',
    'key2.3' => 'val2.3'
  )
  'key3' => 'val3',
  'key4' => array('val4.1', 'val4.2', 'val4.3', 'val4.4', 'val4.5')
);

// Local Configuration
$localConfig = array
(
  'key1' => 'val1_local',
  'key2' => array
  (
    'key2.1' => NULL, // NULL is used to unset this key
    'key2.4' => 'val2.4_local'
  ),
  'key4' => array('val4.1_local')
);

// After merging $globalConfig and $localConfig the goal is 
// that we get this as a result:
$targetConfig = array
(
  'key1' => 'val1_local',
  'key2' => array
  (
    'key2.2' => 'val2.2',
    'key2.3' => 'val2.3',
    'key2.4' => 'val2.4_local'
  ),
  'key3' => 'val3',
  'key4' => array('val4.1_local')
);