Running out of memory

I've got a fairly straight-forward function in a module I'm developing that's running out of memory. It was working fine for quite a while in production, but as the number of website users grew it reached the point where it exhausted the memory available to it. It now consistently fails with errors like: "Fatal error: Out of memory (allocated 33292288) (tried to allocate 207 bytes)".

I'm in a shared hosting environment so I can't increase the memory limit. Nor do I think this is really a solution. The current limit is 90MB and this gets used up pretty quickly. We only have 300 users and not all of these are processed by the function. So I'm looking for some tips on managing memory a bit better.

I've reproduced the code below. It's edited a little. I've added comments so I can refer to sections. I've snipped some irrelevant stuff to keep it short. I've been using the memory_get_usage() function to help me monitor where memory is being used up. I've also got a test site with only a small number of users, on which the function does run to completion.

Section A initialises the function's main variables. At the completion of the execution of this section, some 17MB of memory is in use. This will include all the Drupal stuff as well as my module. So we're well within the memory limit at this point.

Section B is where the memory usage grows and eventually runs out. Very roughly, every five users processed seems to use up an extra 1MB of memory. 270 users can be safely processed. By 290, we run out of memory every time. As far as I can see, this is due to the step at B1, where a user's CiviCRM contact record is loaded and an array with selected user data from this contact record is returned to my function. However, the two variables that hold this data: $contact_data and $result go out of scope with each loop iteration and are set again in the next one. I imagine that any memory they were using could be free'd after each iteration. (Note: this is based on a Java/C++ idea of scope and garbage collection, not PHP - a language in which I'm not an expert.) Yet the memory usage just seems to grow.

Section C which build the actual HTML output, based on the processed data, adds hardly anything to the memory usage.

Questions:
1) Should PHP be garbage collecting (in any way)? It seems to me that when the memory is used up, a lot could then be free'd up as it's no longer required.
2) Is there a way I can force memory to be released after each iteration of the loop in Section B.
3) Can you see any memory leaks in my code?
4) Is there a better way to do this sort of processing? This function doesn't need to be fast, but it does need to use less memory.

Any suggestions much appreciated.

function _rdata_report($full) {
  $output = '';

  // A: initialisation - configure a set of arrays
  $comps = _rdata_comps(); // Map of competition codes to names
  $teams = _rego_teams(); // Map of team codes to names
  $counts = array();
  $players = array();
  $counts['All'] = 0;
  foreach ($comps as $comp => $name) {
    $counts[$comp] = 0;
  }
  foreach ($teams as $team => $name) {
    $counts[$team] = 0;
    $players[$team] = array();
  }

  $uids = _rdata_uids(); // Returns an array of the user id's of all active users
  // B: Load and process user data
  foreach ($uids as $uid) {
    if ($uid == 1) {
      continue;
    }
    $contact_data = _rego_get_contact_data($uid); // B2: Loads user's contact data from CiviCRM (if any)
    $result = _rego_validate($contact_data);
    if ($result['valid']) {
      $counts['All']++;
      $counts[$contact_data['Competition']]++;
      $counts[$contact_data['Team']]++;
      $players[$contact_data['Team']][] = $contact_data['Full Name'];
    }
  }

  // C: Produce output based on user data.
  $output .= '<h3>';
  $output .= 'Total Valid Registrations - ' . $counts['All'] . ' players';
  $output .= '</h3>';
  $output .= '<ul>';
  foreach ($comps as $comp => $compName) {
    $output .= '<li>' . $compName . ' (' . $counts[$comp] . ')';
    $output .= '<ul>';
    foreach ($teams as $team => $teamName) {
      if (substr($team, 0, strlen($comp)) != $comp) {
        continue;
      }
      $output .= '<li>' . $teamName . ' (' . $counts[$team] . ')';
      if ($full && $players[$team]) {
        $output .= '<ul>';
        foreach ($players[$team] as $player) {
          $output .= '<li>' . $player . '</li>';
        }
        $output .= '</ul>';
      }
      $output .= '</li>';
    }
    $output .= '</ul></li>';
  }
  $output .= '</ul>';
  $output .= '<br />';
  $output .= '<br />';

  return $output;
}