[Solved] Automatically replace the first & with a ? question mark.

I have this code below which generates a query string for my url.

But I would like to know is there away that it could automatically replace the first & with a ? question mark.

PHP Code:

$url = '';

if ($this->input->get('directory')) {
   $pos = strrpos($this->input->get('directory'), '/');

   if ($pos) {
      $url .= '&directory=' . urlencode(substr($this->input->get('directory'), 0, $pos));
   }
}

if ($this->input->get('target')) {
    $url .= '&target=' . $this->input->get('target'); 
}

if ($this->input->get('thumb')) {
    $url .= '&thumb=' . $this->input->get('thumb'); 
} 


$data['parent'] = base_url('image_manager' . $url);