<?php
    $themes=array(
        "themes"=>array("hello","world")
    );
    $json=json_encode($themes);
    echo $json;

?>

$.ajax{
   url: "http://my-demo.ifreeweb.net/question/theme_json.php",
   cache: false,
   dataType: 'jsonp',
   jsonp: 'myfun'
}

function myfun(data){
  alert(data.themes);
}