PHP explode at capital letters

<?php
$str = "ExplodeAtCapitalLetters";
$str[0] = strtolower($str[0]);
$pieces = preg_split('/(?=[A-Z])/',$str);
$pieces[0][0] = strtoupper($pieces[0][0]);
print_r($pieces);



Array
(
    [0] => Explode
    [1] => At
    [2] => Capital
    [3] => Letters
)