Manipulating multidimensional data structures in PHP

filter.php

<?php

class KeyError extends Exception {};

function filter_tree($tree, $fields) {
    if (empty($fields))
    {
        return $tree;
    }
    $result = array();
    foreach ($fields as $field)
    {
        $result = recursive_set($result,
                                $field,
                                recursive_get($tree, $field));
    }
    return $result;
}

function recursive_get($tree, $fields)
{
    $get = function($tree, $field) {
        if (!isset($tree[$field]))
        {
            throw new KeyError($field);
        }
        return $tree[$field];
    };
    return array_reduce($fields,
                        $get,
                        $tree);
}

function recursive_set($tree, $fields, $value)
{
    $ref = &$tree;
    foreach ($fields as $field)
    {
        if (!isset($ref[$field]))
            $ref[$field] = array();
        $ref = &$ref[$field];
    }
    $ref = $value;
    return $tree;
}

filtertests.php

<?php

require_once("filter.php");

class FilterTests extends PHPUnit_Framework_TestCase {
    public function testFilter1D() {
        $data = array('foo' => 'bar',
                      'baz' => 'quux');
        $filtered = filter_tree($data,
                                array(array('foo')));
        $this->assertEquals(array('foo' => 'bar'),
                            $filtered);
    }
    public function testFilter2D() {
        $data = array('foo' => array('a' => 1,
                                     'b' => 2),
                      'bar' => 'baz');
        $filtered = filter_tree($data,
                                array(array('foo', 'b')));
        $this->assertEquals(array('foo' => array('b' => 2)),
                            $filtered);
    }
    public function testFilterMultiple() {
        $data = array('foo' => array('a' => 1,
                                     'b' => 2),
                      'bar' => array('a' => array(),
                                     'b' => array('name' => 'MyName',
                                                  'description' => 'Description')),
                      'baz' => array('a' => 123));
        $filtered = filter_tree($data,
                                array(array('foo', 'b'),
                                      array('bar', 'b', 'description'),
                                      array('baz')));
        $this->assertEquals(array('foo' => array('b' => 2),
                                  'bar' => array('b' => array('description' => 'Description')),
                                  'baz' => array('a' => 123)),
                            $filtered);
    }
    public function testFilterNone() {
        $data = array('foo' => array('a' => 1,
                                     'b' => 2),
                      'bar' => array('a' => array(),
                                     'b' => array('name' => 'MyName',
                                                  'description' => 'Description')),
                      'baz' => array('a' => 123));
        $this->assertEquals($data,
                            filter_tree($data, array()));
    }
}