Creating a nested array from items with parent IDs

nested_array.php

function makeNested($source) {
	$nested = array();

	foreach ( $source as &$s ) {
		if ( is_null($s['parent_id']) ) {
			// no parent_id so we put it in the root of the array
			$nested[] = &$s;
		}
		else {
			$pid = $s['parent_id'];
			if ( isset($source[$pid]) ) {
				// If the parent ID exists in the source array
				// we add it to the 'children' array of the parent after initializing it.

				if ( !isset($source[$pid]['children']) ) {
					$source[$pid]['children'] = array();
				}

				$source[$pid]['children'][] = &$s;
			}
		}
	}
	return $nested;
}

/*
$source = array(
    1 => array(
		'name' => 'Parent One',
		'parent_id' => null
	),
	2 => array(
		'name' => 'Parent Two', 
		'parent_id' => null
	),
	3 => array(
		'name' => 'Child One',
		'parent_id' => 1
	),
	4 => array(
		'name' => 'Child Two', 
		'parent_id' => 1
	),
	5 => array(
		'name' => 'Child Three',
		'parent_id' => 2
	),
	6 => array(
		'name' => 'Child Four',
		'parent_id' => 5
	),
	7 => array(
		'name' => 'Child Five', 
		'parent_id' => 2
	),
	8 => array(
		'name' => 'Child Six',
		'parent_id' => 3
	),
	9 => array(
		'name' => 'Child Seven',
		'parent_id' => 6
	)
);

Array
(
    [0] => Array
        (
            [name] => Parent One
            [parent_id] => 
            [children] => Array
                (
                    [0] => Array
                        (
                            [name] => Child One
                            [parent_id] => 1
                            [children] => Array
                                (
                                    [0] => Array
                                        (
                                            [name] => Child Six
                                            [parent_id] => 3
                                        )
                                )
                        )

                    [1] => Array
                        (
                            [name] => Child Two
                            [parent_id] => 1
                        )
                )
        )

    [1] => Array
        (
            [name] => Parent Two
            [parent_id] => 
            [children] => Array
                (
                    [0] => Array
                        (
                            [name] => Child Three
                            [parent_id] => 2
                            [children] => Array
                                (
                                    [0] => Array
                                        (
                                            [name] => Child Four
                                            [parent_id] => 5
                                            [children] => Array
                                                (
                                                    [0] => Array
                                                        (
                                                            [name] => Child Seven
                                                            [parent_id] => 6
                                                        )
                                                )
                                        )
                                )
                        )
                    [1] => Array
                        (
                            [name] => Child Five
                            [parent_id] => 2
                        )
                )
        )
)
*/