curl_multi keep request index on asynchronous response

Question

I found a script to have asynchronous response from curl_multi requests:

function rolling_curl($urls, $callback, $custom_options = null) {

// make sure the rolling window isn't greater than the # of urls
$rolling_window = 100;
$rolling_window = (sizeof($urls) < $rolling_window) ? sizeof($urls) : $rolling_window;

$master = curl_multi_init();
$curl_arr = array();

// add additional curl options here
$std_options = array(CURLOPT_RETURNTRANSFER => true,
CURLOPT_FOLLOWLOCATION => true,
CURLOPT_MAXREDIRS => 5);
$options = ($custom_options) ? ($std_options + $custom_options) : $std_options;
$hosts = array();
// start the first batch of requests
for ($i = 0; $i < $rolling_window; $i++) {
    $hosts[$i]=parse_url($urls[$i], PHP_URL_HOST);

    $ch = curl_init();
    $options[CURLOPT_URL] = $urls[$i];
    curl_setopt_array($ch,$options);
    curl_multi_add_handle($master, $ch);
}

do {
    while(($execrun = curl_multi_exec($master, $running)) == CURLM_CALL_MULTI_PERFORM);
    if($execrun != CURLM_OK)
        break;
    // a request was just completed -- find out which one
    while($done = curl_multi_info_read($master)) {
        $info = curl_getinfo($done['handle']);

        if ($info['http_code'] == 200)  {
            $output = curl_multi_getcontent($done['handle']);

            // request successful.  process output using the callback function.
           $id = array_search(parse_url($info['url'], PHP_URL_HOST),$hosts);
            $callback($output,true, $id);

            // start a new request (it's important to do this before removing the old one)
            if(@$u = $urls[$i++]){
              $ch = curl_init();
            $options[CURLOPT_URL] = $u;  // increment i
            curl_setopt_array($ch,$options);
            curl_multi_add_handle($master, $ch);  
            }




            // remove the curl handle that just completed
            curl_multi_remove_handle($master, $done['handle']);
        } else {
            $callback("",false);
        }
    }
} while ($running);

curl_multi_close($master);
return true;
} 

This is already edited by me because what I was trying to do was keeping the index of the url array input on callback function. For example if input array is

$urls = array(0=> $url0, 1=> $url1, ...);

then I want that when curl request on url1 is completed, $callback is triggered with $id = 1 so that I know how to manage data returned, because I know it is url1. To do this I thought of finding the curl url in the urls array, but sometimes curl follow redirect, so it's not always working. Then I thought of comparing the host (fortunately every url has a different host in my case) and it works: I create a host array when creating curl handles and when request is completed I find the index of the curl host into the array. This is not the best way to get what I want, because with urls with same host it wouldn't work and maybe there is a tricky and more easy way to do that. Which is the best way to get the url index? PS: my callback function is

function callback($output,$success,$id){...}