title: "X-Requested-With" header is set, but not in php
body: <p>The X-Requested-With header is definitely sent:</p>
  
  <pre><code>myApp.config(['$stateProvider', '$urlRouterProvider', '$httpProvider',
            function ($stateProvider, $urlRouterProvider, $httpProvider) {
  
    $httpProvider.defaults.headers.common["X-Requested-With"] = 'XMLHttpRequest';
  
  }]);
  </code></pre>
  
  <p><a href="http://i.stack.imgur.com/OAkR7.png" rel="nofollow">Request Header</a>
  <br><br>
  but in php i can't get the value.</p>
  
  <pre><code>if($_SERVER['X-Requested-With'] != 'XMLHttpRequest') // &lt;-- empty
  {
    die('not allowed');
  }
  </code></pre>