What is the callback function in PHPcURL concurrency?

What is the callback function in PHPcURL concurrency? Curl cURL php concurrency curlcURL php concurrency curl cURL concurrency

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;}

That is, what is the function of row 30th?
Curl_multi_getcontent ($ done ['handle']) I know, what is the role of the following parameter?

Reply to discussion (solution)

Do you know the definition of the custom function callback?
$ Delay is passed in and passed to callback. of course, only you know the meaning.

Well, check what the code of the callback function is written. if you cannot understand it, you can send it.

Do you know the definition of the custom function callback?
$ Delay is passed in and passed to callback. of course, only you know the meaning.

Well, check what the code of the callback function is written. if you cannot understand it, you can send it.

Found... It turned out to be... testing performance...

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

Alas... This is caused by incomplete reading of articles...
Thank you for your attention ~~~