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PHPのjson_encode=>json_decodeで空連想配列が配列に変換される現象

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PHPでJSONを扱う場合、json_decode()の第二引数にtrueを指定して配列として扱う場合がよくあると思います。
これを使っていると json_encode()してjson_decode()で戻すと、空連想配列(stdClass)が配列になってしまうようです。
memcachedなんかにJSONで配列を保存しているような場合には注意ですね。
キャッシュ時と非キャッシュ時で異なる形式のJSONをクライアントに返してしまうなんてことが起こってしまいます。

ソース

$r = array('hoge'=> new \stdClass());
$j = json_encode($r);
echo $j . "\n";
print_r(json_decode($j,true));
echo "\n";

結果

{"hoge":{}}
Array
(
    [hoge] => Array
        (
        )

)
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