Is there any forbidden PHP ?

I have some issues with an eBay API call that will simply not work as expected and I start to wonder if that is maybe because of the WordPress container.

I am unsure, but the culprit might be global

This is the snipped that may cause the problem, I am not sure though:

function buildURLArray ($filterarray) {
  global $urlfilter;
  global $i;
  // Iterate through each filter in the array
  foreach($filterarray as $itemfilter) {
    // Iterate through each key in the filter
    foreach ($itemfilter as $key =>$value) {
      if(is_array($value)) {
        foreach($value as $j => $content) { // Index the key for each value
          $urlfilter .= "&itemFilter($i).$key($j)=$content";
        }
      }
      else {
        if($value != "") {
          $urlfilter .= "&itemFilter($i).$key=$value";
        }
      }
    }
    $i++;
  }
  return "$urlfilter";

$urlfilter will be empty.

Any comment or workaround?