Asynchronous multi Curl request without blocking and rolling window (see https://www.onlineaspect.com/2009/01/26/how-to-use-curl_multi-without-blocking/)

multi_curl.php

<?php

function rolling_curl($urls, $callback, $custom_options = null) {
    // make sure the rolling window isn't greater than the # of urls
    $rolling_window = 5;
    $rolling_window = (sizeof($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) : $std_options;
    // start the first batch of requests
    for ($i = 0; $i < $rolling_window; $i++) {
        $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.
                $callback($output);
                // start a new request (it's important to do this before removing the old one)
                $ch = curl_init();
                $options[CURLOPT_URL] = $urls[$i++];  // 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, $done['handle']);
            } else {
                // request failed.  add error handling.
            }
        }
    } while ($running);
    
    curl_multi_close($master);
    return true;
}

mutli_curl_head.php

<?php
function getHead($urls){
    $results = array();
    // make sure the rolling window isn't greater than the # of urls
    $rolling_window = 5;
    $rolling_window = (sizeof($urls) < $rolling_window) ? sizeof($urls) : $rolling_window;
    $master = curl_multi_init();
    // $curl_arr = array();
    // add additional curl options here
    $options = array(
      CURLOPT_FOLLOWLOCATION => FALSE,
      CURLOPT_RETURNTRANSFER => TRUE,
      CURLOPT_NOBODY => TRUE,
    );
    // start the first batch of requests
    for ($i = 0; $i < $rolling_window; $i++) {
      $ch = curl_init();
      $options[CURLOPT_URL] = array_pop($urls);
      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']);
        $results[$info['url']] = $info;
        $new_url = array_pop($urls);
        if(isset($new_url)){
          $ch = curl_init();
          $options[CURLOPT_URL] = $new_url;
          curl_setopt_array($ch, $options);
          curl_multi_add_handle($master, $ch);
        }
        // remove the curl handle that just completed
        curl_multi_remove_handle($master, $done['handle']);
      }
    } while ($running);
    curl_multi_close($master);
    return $results;
  }