Отправка push-уведомлений через OneSignal с использованием переполнения стека

Я использую следующий код PHP для отправки push-уведомлений через One Signal:

function sendMessage(){
$content = array(
"en" => 'English Message'
);

$fields = array(
'app_id' => $appId,
'included_segments' => array('All'),
'data' => array("foo" => "bar"),
'contents' => $content
);

$fields = json_encode($fields);
print("\nJSON sent:\n");
print($fields);

$ch = curl_init();
curl_setopt($ch, CURLOPT_URL, "https://onesignal.com/api/v1/notifications");
curl_setopt($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json; charset=utf-8',
'Authorization: Basic '.$restKey));
curl_setopt($ch, CURLOPT_RETURNTRANSFER, TRUE);
curl_setopt($ch, CURLOPT_HEADER, FALSE);
curl_setopt($ch, CURLOPT_POST, TRUE);
curl_setopt($ch, CURLOPT_POSTFIELDS, $fields);
curl_setopt($ch, CURLOPT_SSL_VERIFYPEER, FALSE);

$response = curl_exec($ch);
curl_close($ch);

return $response;
}

$response = sendMessage();
$return["allresponses"] = $response;
$return = json_encode( $return);

print("\n\nJSON received:\n");
print($return);
print("\n");

И я получаю следующий ответ:

JSON sent: {"app_id":null,"included_segments":["All"],"data":{"foo":"bar"},"contents":{"en":"English Message"}} JSON received: "{\"allresponses\":\"{\\\"adm_big_picture\\\":null,\\\"adm_group\\\":null,\\\"adm_group_message\\\":null,\\\"adm_large_icon\\\":null,\\\"adm_small_icon\\\":null,\\\"adm_sound\\\":null,\\\"amazon_background_data\\\":false,\\\"android_accent_color\\\":null,\\\"android_group\\\":null,\\\"android_group_message\\\":null,\\\"android_led_color\\\":null,\\\"android_sound\\\":null,\\\"android_visibility\\\":null,\\\"app_id\\\":\\\"63c6c8f7-694b-4c68-abc1-d820d9bbbec1\\\",\\\"big_picture\\\":null,\\\"buttons\\\":null,\\\"canceled\\\":false,\\\"chrome_big_picture\\\":null,\\\"chrome_icon\\\":null,\\\"chrome_web_icon\\\":\\\"\\\",\\\"chrome_web_image\\\":\\\"\\\",\\\"content_available\\\":false,\\\"contents\\\":{\\\"en\\\":\\\"This is a new message.\\\"},\\\"converted\\\":0,\\\"data\\\":null,\\\"delayed_option\\\":\\\"immediate\\\",\\\"delivery_time_of_day\\\":\\\"4:00 PM\\\",\\\"errored\\\":0,\\\"excluded_segments\\\":[],\\\"failed\\\":0,\\\"firefox_icon\\\":\\\"\\\",\\\"headings\\\":{\\\"en\\\":\\\"New Message\\\"},\\\"id\\\":\\\"8d56f592-8f43-461a-94e8-2fe9922ba844\\\",\\\"include_player_ids\\\":null,\\\"included_segments\\\":[\\\"All\\\"],\\\"ios_badgeCount\\\":null,\\\"ios_badgeType\\\":null,\\\"ios_category\\\":null,\\\"ios_sound\\\":null,\\\"isAdm\\\":false,\\\"isAndroid\\\":false,\\\"isChrome\\\":false,\\\"isChromeWeb\\\":true,\\\"isFirefox\\\":true,\\\"isIos\\\":false,\\\"isSafari\\\":true,\\\"isWP\\\":false,\\\"isWP_WNS\\\":false,\\\"large_icon\\\":null,\\\"priority\\\":null,\\\"queued_at\\\":1492523636,\\\"remaining\\\":0,\\\"send_after\\\":1492523636,\\\"small_icon\\\":null,\\\"successful\\\":3,\\\"tags\\\":null,\\\"filters\\\":null,\\\"template_id\\\":null,\\\"ttl\\\":null,\\\"url\\\":\\\"\\\",\\\"web_buttons\\\":null,\\\"wp_sound\\\":null,\\\"wp_wns_sound\\\":null}\"}"

Однако Push-уведомление не появляется на панели инструментов One Signal и не принимается теми, кто подписался.

Может кто-нибудь помочь, пожалуйста :)?