As it says on this page:

"The stream allows access to the body of the resource; the headers are stored in the $http_response_header variable. Since PHP 4.3.0, the headers are available using stream_get_meta_data()."

This one sentence is the only documentation I have found on the mysterious $http_response_header variable, and I'm afraid it's misleading.  It implies that from 4.3.0 onward, stream_get_meta_data() ought to be used in favor of $http_response_header.  

Don't be fooled!  stream_get_meta_data() requires a stream reference, which makes it ONLY useful with fopen() and related functions.  However, $http_response_header can be used to get the headers from the much simpler file_get_contents() and related functions, which makes it still very useful in 5.x.

Also note that even when file_get_contents() and friends fail due to a 4xx or 5xx error and return false, the headers are still available in $http_response_header.

Passing authentication information in the URL as in "https://user:password@example.com" works for HTTP "Basic" access authentication but not for HTTP "Digest" access authentication. You can use the cURL functions for servers requesting HTTP "Digest" access authentication.

HTTP post function;

<?php
function post_it($datastream, $url) { 

$url = preg_replace("@^http://@i", "", $url);
$host = substr($url, 0, strpos($url, "/"));
$uri = strstr($url, "/"); 

      $reqbody = "";
      foreach($datastream as $key=>$val) {
          if (!empty($reqbody)) $reqbody.= "&";
      $reqbody.= $key."=".urlencode($val);
      } 

$contentlength = strlen($reqbody);
     $reqheader =  "POST $uri HTTP/1.1\r\n".
                   "Host: $host\n". "User-Agent: PostIt\r\n".
     "Content-Type: application/x-www-form-urlencoded\r\n".
     "Content-Length: $contentlength\r\n\r\n".
     "$reqbody\r\n"; 

$socket = fsockopen($host, 80, $errno, $errstr);

if (!$socket) {
   $result["errno"] = $errno;
   $result["errstr"] = $errstr;
   return $result;
}

fputs($socket, $reqheader);

while (!feof($socket)) {
   $result[] = fgets($socket, 4096);
}

fclose($socket);

return $result;
}
?>