Cron job to upload files from server to Dropbox

The token you get back after authorizing the app is called an "access token", and is specific to that particular app-user pair. Access tokens for your app don't expire by default, so once the app receives one, you can have it store and re-use it, so it can make API calls in the future for that user without further manual user intervention. 

So, you'll probably want to have your app do something like this each time it runs:

• check if it has an access token stored already
  • if it does, use that existing access token
  • if it doesn't, send the user through the app authorization flow to get an access token, and store the resulting access token
• call the API to upload the file(s)

I don't have a sample of this to share in PHP though. Exactly what mechanism you use to store the access token will depend on what platform and libraries you're using. Keep in mind that the access token does enable access to the account, so you should store it securely.