Create the file php.ini at the root of the project. If you need to increase the PHP memory limit:
memory_limit = 1024M
For a list of recommended PHP configuration settings, see Required PHP settings.
After pushing your file, you can check that the custom PHP configuration has been added to your environment by creating an SSH tunnel to your environment and entering:
cat /etc/php5/fpm/php.ini
Reference: Magento DevDocs - Customize php.ini settings