Change the group permission for the folder
sudo chown -R w3cert /home/w3cert/.composer/cache/repo/https---packagist.org
and the Files folder too
sudo chown -R w3cert /home/w3cert/.composer/cache/files/
I'm assuming w3cert is your username, if not change the 4th parameter to your username.
If the problem still persists try
sudo chown -R w3cert /home/w3cert/.composer
Now, there is a chance that you won't be able to create your app directory, if that happens do the same for your html folder or the folder you are trying to create your laravel project in.
Hope this helps.