Angular - Dados em branco

var App = angular.module('App',[]);

App.controller('AppCtrl',function($scope, $http){
    $http.post("usuarios.php").success(function(data){
        $scope.details = data;
    });
});

<?php

$con = mysqli_connect("127.0.0.1", "root", "", "banco");

$query = "SELECT * from tab_usuarios";
$result = mysqli_query($con, $query);
if(mysqli_num_rows($result) != 0) {

    $i = 0;
    $arr = array('usuarios' => array());
    while($row = mysqli_fetch_assoc($result)){

        $arr['usuarios'][$i]['id'] = utf8_encode($row['id']);
        $arr['usuarios'][$i]['nome'] = utf8_encode($row['nome']);
        $arr['usuarios'][$i]['email'] = utf8_encode($row['email']);
        $i++;            
    }            
    echo json_encode($arr);
?>