マルチバイト文字がそのままのjson_encode ref: http://qiita.com/items/71e1ec7e52b76f467d29

json_jap.php

<?php
function json_jap($obj){
    $str=json_encode($obj);
    $str=preg_replace_callback('{\\\\u([0-9a-f]{4})}',function($m){return mb_convert_encoding(hex2bin($m[1]),'UTF-8','UTF-16');},$str);
    $str=str_replace(['\/','&gt;','&lt;','&apos;','&amp;'],['/','>','<','\'','&'],$str);
    return $str;
}
?>