Как получить данные из такой строки?
json_decode  вернет тебе простой ассоциативный массив,  
и далее просто $ara['result']['date']$tmp = '{"ok":true,"result":{"message_id":45,"chat":{"id":-1001220061819,"title":"Test_channel","username":"testldasasd","type":"channel"},"date":1554636998,"photo":[{"file_id":"AgADBAADzagxG360VFHIPPb3hmpSXDkBLRsABMiOIyUl07R4Y5UCAAEC","file_size":1938,"width":90,"height":90},{"file_id":"AgADBAADzagxG360VFHIPPb3hmpSXDkBLRsABFNMe6LznfP8ZJUCAAEC","file_size":28529,"width":319,"height":320},{"file_id":"AgADBAADzagxG360VFHIPPb3hmpSXDkBLRsABMaa3B3X6gYXZZUCAAEC","file_size":74939,"width":554,"height":555}],"caption":"\u0411\u043b\u043e\u043a\u043d\u043e\u0442 \n\ud83d\udc49http://ali.pub/39kqrl\n\ud83d\udcb0 \u0426\u0435\u043d\u0430: 49,26 \u20bd\n#\u041e\u0444\u0438\u0441\u043d\u044b\u0435_\u0438_\u0448\u043a\u043e\u043b\u044c\u043d\u044b\u0435_\u043f\u0440\u0438\u043d\u0430\u0434\u043b\u0435\u0436\u043d\u043e\u0441\u0442\u0438 \n#\u0417\u0430\u043f\u0438\u0441\u043d\u044b\u0435_\u043a\u043d\u0438\u0433\u0438_\u0438_\u0431\u043b\u043e\u043a\u043d\u043e\u0442\u044b\n#\u0414\u0435\u0448\u0435\u0432\u043b\u0435_300\u0420","caption_entities":[{"offset":11,"length":21,"type":"url"},{"offset":50,"length":34,"type":"hashtag"},{"offset":86,"length":26,"type":"hashtag"},{"offset":113,"length":13,"type":"hashtag"}]}}';
         $tmpArray = json_decode($tmp, true);
          echo $tmpArray['result']['date'];
sandbox.onlinephpfunctions.com/code/c36efec9482f4a...
json_decode  вернет тебе простой ассоциативный массив,  
и далее просто $ara['result']['date']$tmp = '{"ok":true,"result":{"message_id":45,"chat":{"id":-1001220061819,"title":"Test_channel","username":"testldasasd","type":"channel"},"date":1554636998,"photo":[{"file_id":"AgADBAADzagxG360VFHIPPb3hmpSXDkBLRsABMiOIyUl07R4Y5UCAAEC","file_size":1938,"width":90,"height":90},{"file_id":"AgADBAADzagxG360VFHIPPb3hmpSXDkBLRsABFNMe6LznfP8ZJUCAAEC","file_size":28529,"width":319,"height":320},{"file_id":"AgADBAADzagxG360VFHIPPb3hmpSXDkBLRsABMaa3B3X6gYXZZUCAAEC","file_size":74939,"width":554,"height":555}],"caption":"\u0411\u043b\u043e\u043a\u043d\u043e\u0442 \n\ud83d\udc49http://ali.pub/39kqrl\n\ud83d\udcb0 \u0426\u0435\u043d\u0430: 49,26 \u20bd\n#\u041e\u0444\u0438\u0441\u043d\u044b\u0435_\u0438_\u0448\u043a\u043e\u043b\u044c\u043d\u044b\u0435_\u043f\u0440\u0438\u043d\u0430\u0434\u043b\u0435\u0436\u043d\u043e\u0441\u0442\u0438 \n#\u0417\u0430\u043f\u0438\u0441\u043d\u044b\u0435_\u043a\u043d\u0438\u0433\u0438_\u0438_\u0431\u043b\u043e\u043a\u043d\u043e\u0442\u044b\n#\u0414\u0435\u0448\u0435\u0432\u043b\u0435_300\u0420","caption_entities":[{"offset":11,"length":21,"type":"url"},{"offset":50,"length":34,"type":"hashtag"},{"offset":86,"length":26,"type":"hashtag"},{"offset":113,"length":13,"type":"hashtag"}]}}';
         $tmpArray = json_decode($tmp, true);
          echo $tmpArray['result']['date'];
sandbox.onlinephpfunctions.com/code/c36efec9482f4a...