<?php

$default = array(
    'a' => array(
        'a' => 'a',
        'b' => 'b',
        'c' => 'c',
    ),
);

$user = array(
    'b' => 1,
    'a' => array(
        'b' => 'something',
        'c' => 'something else',
        'd' => 'd',
    ),
    'c' => 2);

function fill_in($user, $default) {
    fill_in_aux($user, $default);
    return $default;
}

function fill_in_aux($user, &$default) {
    foreach ($user as $key => $value) {
        if (is_array($value)) {
            fill_in_aux($user[$key], $default[$key]);
        } else {
            $default[$key] = $value;
        }
    }
}

print_r(fill_in($user, $default));