If you are using php 5.4 or above, there does not seem to be a php_http.dll file to include in your extensions library (Unless someone can find one that I missed??). 
The only one that I could find generated errors on starting up the Apache server after updating the php.ini configuration file to include the extension.
Fear not however, as there seems to be a GitHub Project which provides the functionality within a class, rather than an extension. Click here to find the required class.
If you save this class in your project and call like so;
include_once('HttpRequest.php'); //where HttpRequest.php is the saved file
$url= 'http://www.google.com/';
$r = new HttpRequest($url, "POST");
var_dump($r->send());
Failing that, it would seem that the only other viable option would be to compile the .dll yourself from the source here  :(
Otherwise, another option would be to use cURL instead. cURL provides most (if not all) of the functionality of the httpRequest. 
A simple example of this would be;
$url = "http://www.google.com/";        
$ch = curl_init(); 
curl_setopt($ch, CURLOPT_URL, $url); 
curl_setopt($ch, CURLOPT_HEADER, TRUE); 
curl_setopt($ch, CURLOPT_NOBODY, TRUE); // remove body 
$head = curl_exec($ch); 
$httpCode = curl_getinfo($ch, CURLINFO_HTTP_CODE); 
curl_close($ch); 
var_dump($head);
More details and better examples can be found on the php website Here
I hope this helps answer your question, rather than leave you with more...