Ajax request to php from angular js

I am trying make an ajax request to php from angular js. But I am not getting the data I have sent. Please let me know where I am going wrong.

The following is the ajax call.

<script type="text/javascript">

    angular.module('app', []).controller("MyController", function($scope, $http) {

    var req = {
        method: 'POST',
        url: 'pujastrail/www/rest/get.php',
        headers: {
            'Content-Type': "application/x-www-form-urlencoded; charset=utf-8"
        },
        data: { lang: "fr" }
    }
    $http(req).success(function(data, status, headers, config){alert("done"+data);}).error(function(data, status, headers, config){alert("error"+res);});

        });
</script>

The following is the php code.

<?php
    if (isset($_POST['lang']) && !empty($_POST['lang'])) {
        $lang = $_POST['lang'];//this needs to be sent back to js file
    } else {
        $lang = "eRROR";// but this is being sent back to the js file
    }

    echo (json_encode($lang));
?>

I have tried this using $_REQUEST also but it didnot work.