how to send a notification to millions of android

I am a new developer of Android app. I am sending notifications from PHP code to the android devices using GCM. I made an array of all device ids and send but the problem is when i send more than a thousand devices . I found internal server error. My code is below

function _send_notification($registatoin_ids = '', $message = '') {
    $url = 'https://android.googleapis.com/gcm/send';
    $fields = array(
        'registration_ids' => $registatoin_ids,
        'data' => $message,
    );
    //pr($fields);
    $headers = array(
        'Authorization: key=' . $this->GOOGLE_API_KEY,
        'Content-Type: application/json',
        'Connection: keep-alive',
        'Keep-Alive: 300'
    );
    $ch = curl_init();
    // Set the url, number of POST vars, POST data
    curl_setopt($ch, CURLOPT_URL, $url);
    curl_setopt($ch, CURLOPT_POST, true);
    curl_setopt($ch, CURLOPT_HTTPHEADER, $headers);
    curl_setopt($ch, CURLOPT_RETURNTRANSFER, true);
    curl_setopt($ch, CURLOPT_SSL_VERIFYHOST, 0);
    //curl_setopt($ch, CURLOPT_CONNECT_ONLY, 1);
    curl_setopt($ch, CURLOPT_TIMEOUT, 10000); // drop connection after 10000 seconds
    // Disabling SSL Certificate support temporarly
    curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, false);
    curl_setopt($ch, CURLOPT_POSTFIELDS, json_encode($fields));
    // Execute post
    $result = curl_exec($ch);
    if ($result === FALSE) {
        die('Curl failed: ' . curl_error($ch));
    }
    // Close connection
    curl_close($ch);
    return $result; //exit;
}

Please help me out

The maximum number of registration ids you can send is 1,000. You'll need to break it into multiple curl calls with groupings of 1k registration ids each.

function _send_notification($registration_ids = '', $message = '') {
    $url = 'https://android.googleapis.com/gcm/send';
    $groups = array_chunk($registration_ids, 1000);

    foreach($groups as $group) {
        $fields = array(
            'registration_ids' => $group,
            'data' => $message,
        );
        // ...
        // rest of your curl code
    }
}