<?php
$content='"{\"orderId\":\"ORDER-294\",\"paymentState\":\"CANCELED\",\"signature\":\"naK+I79FduGkXFHpVT\/aJT31EfQ=\",\"message\":\"\u041a\u043b\u0438\u0435\u043d\u0442 \u043d\u0435 \u0437\u0430\u0432\u0435\u0440\u0448\u0438\u043b \u043e\u043f\u043b\u0430\u0442\u0443\",\"storeId\":\"158CC6EE85DA472AB3FB\"}"';
$res=json_decode(json_decode($content), true);
print_r($res);
/*Array
(
[orderId] => ORDER-294
[paymentState] => CANCELED
[signature] => naK+I79FduGkXFHpVT/aJT31EfQ=
[message] => Клиент не завершил оплату
[storeId] => 158CC6EE85DA472AB3FB
)
*/
