<?php
$arr = array();
$str = "billing[firstname]";
$val = "Joshua";

$parts = explode("[", $str);
$arr_ref =& $arr;
foreach ($parts as $part) {
    $part = rtrim($part, "]");
    print $part."\n";
    $arr_ref =& $arr_ref[$part];
}
$arr_ref = $val;
unset($arr_ref);

print_r($arr);