How to Send push notifications using One Signal + PHP + Server API?

You can always refer to the official docs:

https://documentation.onesignal.com/reference#section-example-code-create-notification

• 'app_id' is currently known as (OneSignal App ID) in the OneSignal Settings->Keys and IDs

• in 'Authorization: Basic xxx...' past the "REST API Key" just below the App ID

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

    $fields = array(
        'app_id' => "xxxxxxxx-xxxx-xxxx-xxxx-xxxxxxxxxxxxx",
        '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($ch, CURLOPT_HTTPHEADER, array('Content-Type: application/json; charset=utf-8',
                                               'Authorization: Basic xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx'));
    curl_setopt($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");
?>

I see you have set isAndroid=true, but OneSignal is returning an error that shows that the application with ID eec33e8e-5774-4b74-9aae-37370778c4b2 does not have Android notifications enabled.

Make sure your app ID is correct, and if it is, that Android notifications are enabled in your OneSignal settings.