You will need to make sure that the directory you are writing to allows for www-data to write to it. Typically, you will want to put that directory in a place that is away from other files etc…

sudo chown  www-data:www-data <DIRNAME>
sudo chmod -R  g+w <DIRNAME>

For changing owner  :

sudo chown -R  <owner name > <dir name>

For globally set write permission fro www-data user :

sudo chown -R www-data:www-data /var/www
sudo chmod -R g+rw /var/www