var request;
 
 try {
    request = new XMLHttpRequest();
  } catch (trymicrosoft) {
    try {
      request = new ActiveXObject("Msxml2.XMLHTTP");
    } catch (othermicrosoft) {
      try {
        request = new ActiveXObject("Microsoft.XMLHTTP");
      } catch (failed) {
        request = false;
      }
    }
  }
  if (!request)
  alert("Error initializing XMLHttpRequest!");


   request.onreadystatechange=function()
    {
        if (request.readyState==4) // if the data transfer has been completed
        {
            if(request.status==200)  // if server response equal "200 OK"
            {
                document.getElementById("resp").innerHTML=request.responseText; // recording results in the field
 
            }
 
            else //if server response not equal "200 OK"
            {
                alert('Error while creating connection with server');// error message
            }
        }
 
        else //if the data from the server isn't loaded that
        {
            document.getElementById("resp").innerHTML='Loading...'; // recording notice in the field
        }
    }

  request.open("POST", '4.php', true);
  s='123456';
  request.send(s);

   <?php
   var_dump($_POST);
   ?>   

Request URL:http://localhost/tim/4.php
Request Method:POST
Status Code:200 OK
Request Headersview source
Accept:*/*
Accept-Charset:windows-1251,utf-8;q=0.7,*;q=0.3
Accept-Encoding:gzip,deflate,sdch
Accept-Language:ru-RU,ru;q=0.8,en-US;q=0.6,en;q=0.4
Connection:keep-alive
Content-Length:3
Content-Type:application/xml
Host:localhost
Origin:http://localhost
Referer:http://localhost/tim/5.php
User-Agent:Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.11 (KHTML, like Gecko) Chrome/23.0.1271.97 Safari/537.11
Request Payload
123456
Response Headersview source
Connection:Keep-Alive
Content-Length:44
Content-Type:text/html
Date:Sat, 29 Dec 2012 12:14:13 GMT
Keep-Alive:timeout=5, max=94
Server:Apache/2.4.3 (Win32) OpenSSL/1.0.1c PHP/5.4.7
X-Powered-By:PHP/5.4.7