mkdir -p 的な（こういうの便利そう）

array_data_store.php

<?php

class ArrayDataStore extends ArrayIterator {
	protected $separator = '.';

	public function set_separator($separator) {
		$this->separator = $separator;
		return $this;
	}

	public function get($key, $default = null) {
		$parent = $this->get_parent_array($key, $last_ns);
		return !is_null($parent) && array_key_exists($last_ns, $parent) ? $parent[$last_ns] : $default;
	}

	public function has_key($key) {
		$parent = $this->get_parent_array($key, $last_ns);
		return !is_null($parent) && array_key_exists($last_ns, $parent);
	}

	public function set($key, $value) {
		$parent =& $this->get_parent_array($key, $last_ns, true);
		$parent[$last_ns] = $value;

		return $this;
	}
	
	public function set_default($key, $value) {
		$parent =& $this->get_parent_array($key, $last_ns, true);

		if (!array_key_exists($last_ns, $parent))
			$parent[$last_ns] = $value;

		return $this;
	}
	
	public function set_defaults(array $defaults) {
		foreach ($defaults as $key => $value)
			$this->set_default($key, $value);

		return $this;
	}

	public function delete($key) {
		$parent =& $this->get_parent_array($key, $last_ns);

		if (!is_null($parent) && array_key_exists($last_ns, $parent))
			unset($parent[$last_ns]);

		return $this;
	}

	protected function &get_parent_array($key, &$last_ns, $create_if_not_exist = false) {
		$current =& $this;

		$ns = explode($this->separator, $key);
		$last_ns = array_pop($ns);

		foreach ($ns as $k) {
			if (!array_key_exists($k, $current) || !is_array($current[$k])) {
				if ($create_if_not_exist) {
					$current[$k] = array();
				} else {
					$null = null;
					return $null;
				}
			}

			$current =& $current[$k];
		}

		return $current;
	}
}

array_data_store_example.php

<?php

$a = new ArrayDataStore();

$a['nyan1'] = 'nyan1';

$a['nyan2'] = array(
	'nyan3' => 'nyan3'
);

$a->set('nyan4.nyan5.nyan6', 'nyan6');

$a->set('nyan7.nyan8.nyan9', array(
	'nyan10' => 'nyan10'
));

$a->set('nyan7.nyan8.nyan9.nyan11', 'nyan11');

var_dump($a->get('nyan2'));

// array(1) {
//   ["nyan3"]=>
//   string(5) "nyan3"
// }

var_dump($a->get('nyan4.nyan5.nyan6'));

// string(5) "nyan6"

var_dump($a->get('nyan7.nyan8.nyan9.nyan10'));

// string(6) "nyan10"

var_dump($a->get('nyan7.nyan8.nyan9'));

// array(2) {
//   ["nyan10"]=>
//   string(6) "nyan10"
//   ["nyan11"]=>
//   string(6) "nyan11"
// }

var_dump($a->get('opopo.opopo'));

// NULL

var_dump((array) $a);

// array(4) {
//   ["nyan1"]=>
//   string(5) "nyan1"
//   ["nyan2"]=>
//   array(1) {
//     ["nyan3"]=>
//     string(5) "nyan3"
//   }
//   ["nyan4"]=>
//   array(1) {
//     ["nyan5"]=>
//     array(1) {
//       ["nyan6"]=>
//       string(5) "nyan6"
//     }
//   }
//   ["nyan7"]=>
//   array(1) {
//     ["nyan8"]=>
//     array(1) {
//       ["nyan9"]=>
//       array(2) {
//         ["nyan10"]=>
//         string(6) "nyan10"
//         ["nyan11"]=>
//         string(6) "nyan11"
//       }
//     }
//   }
// }

foreach ($a as $key => $value)
	var_dump($key);

// string(5) "nyan1"
// string(5) "nyan2"
// string(5) "nyan4"
// string(5) "nyan7"

$a->delete('nyan1')
	->delete('nyan2')
	->delete('nyan4')
	->delete('nyan7.nyan8.nyan9.nyan10');

var_dump((array) $a);

// array(1) {
//   ["nyan7"]=>
//   array(1) {
//     ["nyan8"]=>
//     array(1) {
//       ["nyan9"]=>
//       array(1) {
//         ["nyan11"]=>
//         string(6) "nyan11"
//       }
//     }
//   }
// }

var_dump($a->has_key('nyan7.nyan8.nyan9.nyan10'));

// bool(false)

var_dump($a->has_key('nyan7.nyan8.nyan9.nyan11'));

// bool(true)

var_dump($a->has_key('nyan7.nyan8.nyan9.nyan11.nyan12'));

// bool(false)

var_dump($a->has_key('nyan0'));

// bool(false)

$a->set('nyan0', null);

var_dump($a->has_key('nyan0'));

// bool(true)

$a->delete('nyan0');

var_dump($a->has_key('nyan0'));

// bool(false)