Not created the app.txt

I'm not using workspace I'm using my own text editor. So my problem is the app.txt it will not be created. That's why i tried to create it, but does not work either. I'm using Mac OSx Pls send a solution, because i can't go any further. Thank's everything. My error. Fatal error: Uncaught UnexpectedValueException: The stream or file "app.txt" could not be opened: failed to open stream: Permission denied in /Applications/XAMPP/xamppfiles/htdocs/teamtreehouse/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php:107 Stack trace: #0 /Applications/XAMPP/xamppfiles/htdocs/teamtreehouse/vendor/monolog/monolog/src/Monolog/Handler/AbstractProcessingHandler.php(37): Monolog\Handler\StreamHandler->write(Array) #1 /Applications/XAMPP/xamppfiles/htdocs/teamtreehouse/vendor/monolog/monolog/src/Monolog/Logger.php(337): Monolog\Handler\AbstractProcessingHandler->handle(Array) #2 /Applications/XAMPP/xamppfiles/htdocs/teamtreehouse/vendor/monolog/monolog/src/Monolog/Logger.php(392): Monolog\Logger->addRecord(300, 'Foo', Array) #3 /Applications/XAMPP/xamppfiles/htdocs/teamtreehouse/index.php(12): Monolog\Logger->addWarning('Foo') #4 {main} thrown in /Applications/XAMPP/xamppfiles/htdocs/teamtreehouse/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php on line 107

My code :<?php

require __DIR__ . '/vendor/autoload.php';

date_default_timezone_get('Hungary/Budapest');

use Monolog\Logger;
use Monolog\Handler\StreamHandler;

$log = new Logger('name'); 
$log->pushHandler(new StreamHandler('app.txt', Logger::WARNING));
$log->addWarning('Foo');



echo 'Hello World!';