javascript - Ajax returning blank response -

i'm working on simple script want see request , response sent server , recieved @ client via ajax. server returning status 500. doing wrong? below script.

javascript:

<script> function loginjs() {  var xmlhttp; if (window.xmlhttprequest) {// code ie7+, firefox, chrome, opera, safari xmlhttp=new xmlhttprequest(); } else {// code ie6, ie5 xmlhttp=new activexobject("microsoft.xmlhttp"); }  xmlhttp.open("get","http://my-website.com/ajax_exp.php",true);  xmlhttp.onreadystatechange=function() {     alert("state "+xmlhttp.readystate+" status "+xmlhttp.status);  if (xmlhttp.readystate==4 && xmlhttp.status==200)  {     alert(xmlhttp.responsetext);  } } xmlhttp.send(); } </script> 

ajax_exp.php

<?php  header('access-control-allow-origin: http://my-website.com/ajax_exp.php'); add_action('wp_ajax_my_action',        'my_action'); add_action('wp_ajax_nopriv_my_action', 'my_action');  function my_action() { $username = 'username'; $password = 'password';  echo $username; die(); }   ?> 

1) try removing

 header('access-control-allow-origin: http://my-website.com/ajax_exp.php');  add_action('wp_ajax_my_action',        'my_action');  add_action('wp_ajax_nopriv_my_action', 'my_action'); 

replacing with

 my_action(); 

you can trace error;

2) check console log javascript errors.