I am using Google Cloud Messaging from last one year with PHP as server side script and it is working fine on all app until I hasn't sent notification to one of the may application for one month then after that while sending notification to same app I gets blank response like
{"status":[],"0":"\n\n
so I assumed that google may have deactivated my API key so I regenerated my API key from developer console and replaced regenerated key with old key on server side and at that time it had worked I was able to send message again but after two week again I am getting blank response like given above and this time I was sent notification to same app everyday.