如何在一个查询中使用多个CURLOPT_USERAGENT？

Question

I have the following code which currently use only one user agent: Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1a2pre) Gecko/2008073000 Shredder/3.0a2pre ThunderBrowse/3.2.1.8.

My question is how I can use multiple user agents in one time? It should change user agent if the current one will not pass.

function file_get_contents_curl($url) {
    $ch = curl_init();
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_FOLLOWLOCATION, true);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_USERAGENT, 'Mozilla/5.0 (X11; U; Linux i686; en-US; rv:1.9.1a2pre) Gecko/2008073000 Shredder/3.0a2pre ThunderBrowse/3.2.1.8');
    $html = curl_exec($ch);
    curl_close($ch);
    return $html;
}