PHP cURL并发里的callback那行是什么功能？
function rolling_curl($urls, $delay) {<br>
$queue = curl_multi_init();<br>
$map = array();<br>
<br>
foreach ($urls as $url) {<br>
$ch = curl_init();<br>
<br>
curl_setopt($ch, CURLOPT_URL, $url);<br>
curl_setopt($ch, CURLOPT_TIMEOUT, 1);<br>
curl_setopt($ch, CURLOPT_RETURNTRANSFER, 1);<br>
curl_setopt($ch, CURLOPT_HEADER, 0);<br>
curl_setopt($ch, CURLOPT_NOSIGNAL, true);<br>
<br>
curl_multi_add_handle($queue, $ch);<br>
$map[(string) $ch] = $url;<br>
}<br>
<br>
$responses = array();<br>
do {<br>
while (($code = curl_multi_exec($queue, $active)) == CURLM_CALL_MULTI_PERFORM) ;<br>
<br>
if ($code != CURLM_OK) { break; }<br>
<br>
// a request was just completed -- find out which one<br>
while ($done = curl_multi_info_read($queue)) {<br>
<br>
// get the info and content returned on the request<br>
$info = curl_getinfo($done['handle']);<br>
$error = curl_error($done['handle']);<br>
$results = callback(curl_multi_getcontent($done['handle']), $delay);<br>
$responses[$map[(string) $done['handle']]] = compact('info', 'error', 'results');<br>
<br>
// remove the curl handle that just completed<br>
curl_multi_remove_handle($queue, $done['handle']);<br>
curl_close($done['handle']); <div class="clear">
</div>
