- var_dump: documentation ( source)
- preg_replace: documentation ( source)
<?php
$str="prijs=0,209&orderby=price&order=undefined&posts_per_page=undefined";
$repl=['/=/', '/,/', '/&/'];
$with=['[', '],[', ']],['];
$str='[' . preg_replace($repl, $with, $str) . ']';
var_dump($str);