/**
 * setURL
 *
 * Modifies the current URL and redirects the browser
 *
 * @param string key The name of the parameter to set
 * @param mixed value The value to set the parameter to
 * @return void
 * @author Dom Hastings
 */
function setURL(key, value) {
  // set up the url separators
  var separator = {
    // site.url/controller/action/key1:value1/key2:value2
    'key': '/',
    'value': ':'
  }
 
  // get the current url
  var url = window.location.href;
  // check if the specified key already exists
  var exists = url.indexOf(separator.key + key + separator.value);
 
  // if it does
  if (exists > -1) {
    // find the next separator.key
    var last = url.indexOf(separator.key, exists + 1);
 
    // if there is one
    if (last > -1) {
      // replcae the existing value with the one passed
      url = url.substr(0, exists) + separator.key + key + separator.value + escape(value) + url.substr(last);
 
    // if not
    } else {
      // just append it
      url = url.substr(0, exists) + separator.key + key + separator.value + escape(value);
    }
 
  // if it's not already in there
  } else {
    // if the URL doesn't end with a separator.key
    if (url.substr(-1) != separator.key) {
      // append it
      url += separator.key;
    }
 
    // append the value
    url += key + separator.value + escape(value);
  }
 
  // set the url
  window.location.href = url;
}

<!-- Items per page: -->
<select onchange="setURL('limit', this.value)">
  <option value="10"<?=($params['named']['limit'] == '10') ? ' selected="selected"' : ''?>>10</option>
  <option value="20"<?=($params['named']['limit'] == '20') ? ' selected="selected"' : ''?>>20</option>
  <option value="50"<?=($params['named']['limit'] == '50') ? ' selected="selected"' : ''?>>50</option>
  <option value="100"<?=($params['named']['limit'] == '100') ? ' selected="selected"' : ''?>>100</option>
</select>
 
<!-- Page: -->
<select onchange="setURL('page', this.value)">
<?php for ($i = 1; $i <= $paginator->counter(array('format' => '%pages%')); $i++): ?>
  <option value="<?=$i?>"<?=($params['named']['page'] == $i) ? ' selected="selected"' : ''?>><?=$i?></option>
<?php endfor ?>
</select>