- var_dump: documentation ( source)
- parse_str: documentation ( source)
<?php
$str = 'data%5B0%5D%5BfirstItem%5D=true&data%5B0%5D%5BsubItem%5D=true&data%5B0%5D%5BsecondItem%5D=true&data%5B0%5D%5BnumItems%5D=1&data%5B0%5D%5Bitems%5D%5B%5D=Apples';
$array = [];
parse_str($str, $array);
var_dump($array);