You can upload content to a droplet on your account by using SFTP. Many FTP clients including Filezilla provide SFTP support. To set up the connection you would use the following settings:

Protocol: SFTP
Port: 22
User: root
Password: [your droplet’s root password]

You will start out in /root when logging in this way but can change up a directory and access any location on your filesystem. By default for the PHP portion of your app you will most likely be using the default apache web root of /var/www/html