PhP script that pushes messages to Android using Onesignal returns false

Question

I`m trying to send push notifications to android users using the following script:

<?PHP
function sendMessage(){
    $content = array(
        "en" => 'Testing Message'
        );

    $fields = array(
        'app_id' => ".....",
        'included_segments' => array('All'),
        'data' => array("foo" => "bar"),
        'large_icon' =>"ic_launcher_round.png",
        '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($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json; charset=utf-8',
                                               'Authorization: Basic .....'));
    curl_setopt($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");
?>

However json responds "false" when running it:

JSON sent

{
    "app_id":".....",
    "included_segments":["All"],
    "data":{"foo":"bar"},
    "large_icon":"ic_launcher_round.png",
    "contents":{"en":"Testing Message"}
} 

JSON received

{
    "allresponses":false
}

What does that message mean? APP ID and REST API KEy are correct.