Post (XMLHttpRequest) javascript does not work

Personal I'm developing an extension for Chrome, but the post part is not working. The information does not arrive. will the problem be the extension.

chrome.extension.onMessage.addListener(function(request, sender) {
if (request.action === "getSource") {
    var message, data, xhr;

    message = document.querySelector("#message");
    data = request.source;

    message.innerText = "Enviando ao servidor...";

    var postUrl = 'webservice.php';

    xhr  = new XMLHttpRequest();
    xhr.open('POST', postUrl, true);
    xhr.onreadystatechange = function() {
        if(xhr.readyState === 4) {
            if (xhr.status === 200) {
                message.innerText = "Resposta do servidor: " + xhr.responseText;
            } else {
                message.innerText = "Err: " + xhr.status;
            }
        }
      };

    //Enviando dados como RAW
    xhr.send(request.source);
     }
   });


function onWindowLoad() {

 var message = document.querySelector('#message');

 chrome.tabs.executeScript(null, {
 file: "getPagesSource.js"
 }, function() {

    if (chrome.runtime.lastError) {
    message.innerText = 'There was an error injecting script : \n' +   chrome.runtime.lastError.message;
   }
   });

    }

    window.onload = onWindowLoad;

webservice.php

<?php

 $rt = $_POST['data'];
 $fp = fopen("bloco1.txt", "a");


 $escreve = fwrite($fp, '$rt');

 // Fecha o arquivo
 fclose($fp);

 ?>