Getting a Fatal error

This is my code. I've checked multiple times but I'm getting a fatal error for some reason.

<?php
require __DIR__ . '/vendor/autoload.php';
date_default_timezone_set('America/Los_Angeles');

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

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

$log->addWarning('Oh Noes');
echo 'Hello World';

Error message :

Fatal error: Uncaught exception 'UnexpectedValueException' with message 'The stream or file "app.txt" could not be opened: failed to open stream: Permission denied' in /home/treehouse/workspace/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php:87 Stack trace: #0 /home/treehouse/workspace/vendor/monolog/monolog/src/Monolog/Handler/AbstractProcessingHandler.php(37): Monolog\Handler\StreamHandler->write(Array) #1 /home/treehouse/workspace/vendor/monolog/monolog/src/Monolog/Logger.php(289): Monolog\Handler\AbstractProcessingHandler->handle(Array) #2 /home/treehouse/workspace/vendor/monolog/monolog/src/Monolog/Logger.php(341): Monolog\Logger->addRecord(300, 'Oh Noes', Array) #3 /home/treehouse/workspace/index.php(11): Monolog\Logger->addWarning('Oh Noes') #4 {main} thrown in /home/treehouse/workspace/vendor/monolog/monolog/src/Monolog/Handler/StreamHandler.php on line 87

Scratching my head on this one...