PHP multiselect and page navigation

[career]

Good day

Need a little help or advice.

How not to show long list of forum ids when changing pages?
Because now when choose several forums and press page 2, the link is
test.php?do=show&pp=20&page2&forum[]=91&forum[]=95&forum[]=99.
Is there a way I can hide from url such string '&forum[]=91&forum[]=95&forum[]=99' and keep work page navigation?

For one of my new pages I use Multiselect, i mean something like this in template

<label for="select_forum">Choose forum</label>
<select name="forum[]" id="select_forum" size="4" style="display:block" multiple="multiple">
<option value="0" $noforumselected>All forums</option>
<option value="91">Test forum1 with forumid 91</option>
<option value="95">Test forum2 with forumid 95</option>
<option value="99">Test forum2 with forumid 99</option>						
</select>

This code i put into form

<form action="test.php?do=show" method="post">
<input type="hidden" name="s" value="$session[sessionhash]" />
<input type="hidden" name="page" value="$pagenumber" />
<input type="hidden" name="pp" value="$perpage" />
<input type="hidden" name="securitytoken" value="$bbuserinfo[securitytoken]" />
<input type="hidden" name="do" value="viewthreads" />

<label for="select_forum">Choose forum</label>
<select name="forum[]" id="select_forum" size="4" style="display:block" multiple="multiple">
<option value="0" $noforumselected>All forums</option>
<option value="91">Test forum1 with forumid 91</option>
<option value="95">Test forum2 with forumid 95</option>
<option value="99">Test forum2 with forumid 99</option>						
</select>

</form>

In test.php i have required code for working page navigation

PHP Code:


.............
.............

// ###############################
if ($_REQUEST['do'] == 'show')
{
    $vbulletin->input->clean_array_gpc('r', array(
        'perpage'    => TYPE_UINT,
        'pagenumber' => TYPE_UINT,
        'daysprune'  => TYPE_INT,
        'sortfield'  => TYPE_NOHTML,
        'sortorder'  => TYPE_NOHTML,
        'forum'    => TYPE_ARRAY, 
    ));


............
............

$sortfield  =& $vbulletin->GPC['sortfield'];
$perpage    =& $vbulletin->GPC['perpage'];
$pagenumber =& $vbulletin->GPC['pagenumber'];
$daysprune  =& $vbulletin->GPC['daysprune'];
$forum    =& $vbulletin->GPC['forum'];

............
............

$sorturl = "test.php?" . $vbulletin->session->vars['sessionurl'] . "do=show&forum[]=" . implode('&forum[]=', $vbulletin->GPC['forum']) . "&pp=$perpage&daysprune=$daysprune";

$pagenav = construct_page_nav($pagenumber, $perpage, $totalthreads, $sorturl . "&sort=$sortfield" . iif(!empty($vbulletin->GPC['sortorder']), "&order=" . $vbulletin->GPC['sortorder']));