PHP cURL并发里的callback那行是什么功能？

function rolling_curl($urls, $delay) {    $queue = curl_multi_init();    $map = array();     foreach ($urls as $url) {        $ch = curl_init();         curl_setopt($ch, CURLOPT_URL, $url);        curl_setopt($ch, CURLOPT_TIMEOUT, 1);        curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);        curl_setopt($ch, CURLOPT_HEADER, 0);        curl_setopt($ch, CURLOPT_NOSIGNAL, true);         curl_multi_add_handle($queue, $ch);        $map[(string) $ch] = $url;    }     $responses = array();    do {        while (($code = curl_multi_exec($queue, $active)) == CURLM_CALL_MULTI_PERFORM) ;         if ($code != CURLM_OK) { break; }         // a request was just completed -- find out which one        while ($done = curl_multi_info_read($queue)) {             // get the info and content returned on the request            $info = curl_getinfo($done['handle']);            $error = curl_error($done['handle']);            $results = callback(curl_multi_getcontent($done['handle']), $delay);            $responses[$map[(string) $done['handle']]] = compact('info', 'error', 'results');             // remove the curl handle that just completed            curl_multi_remove_handle($queue, $done['handle']);            curl_close($done['handle']);        }         // Block for data in / output; error handling is done by curl_multi_exec        if ($active > 0) {            curl_multi_select($queue, 0.5);        }     } while ($active);     curl_multi_close($queue);    return $responses;}

function callback($data, $delay) {    preg_match_all('/<h3>(.+)<\/h3>/iU', $data, $matches);    usleep($delay);    return compact('data', 'matches');}