preg_replace to build query string like array from php like array

I think this won't work with regex. I wrote a little recursive function that parses your string to an array. This way you won't have to use parse_str() afterwards. I don't think this is the most elegant way, but it works. It can probably be improved.

$str = "B=val4, A = [val1, key=val2, val3], C=val5";
$i = 0;

function parse(&$i, &$str) {
    $array = array();
    $buffer = "";
    $key = "";

    while ($i < strlen($str)) {
        switch ($str[$i]) {
            case " ": //ignore spaces
                break;
            case "[": //call recursive function
                $i++;
                $buffer = parse($i, $str);
                break;
            case "]": //return sub-array
                $key ? $array[$key] = $buffer : $array[] = $buffer; // add last sub-element to array
                return $array;
                break;
            case "=": //set key and reset buffer
                $key = $buffer;
                $buffer = "";
                break;
            case ",": //add to array
                $key ? $array[$key] = $buffer : $array[] = $buffer;
                $key = "";
                $buffer = "";
                break;
            default : // add char to buffer
                $buffer .= $str[$i];
                break;
        }
        $i++;
    }
    $key ? $array[$key] = $buffer : $array[] = $buffer; // add last element to array
    return $array;
}

$array = parse($i, $str);
echo "<pre>";
print_r($array);

Output:

Array
(
    [B] => val4
    [A] => Array
        (
            [0] => val1
            [key] => val2
            [1] => val3
        )

    [C] => val5
)

Hope this helps.