PHPでjsonをレスポンスするにはjson_encode関数を呼び出す前にheader(“Content-Type: application/json”);を実行する
http://stackoverflow.com/questions/13709812/php-generated-json-variables-not-showing-in-d3-js
http://bl.ocks.org/mbostock/3883245
http://qiita.com/items/db4b46d6802d7295cb67