The user only has to authorize your app once. After that, you can just keep reusing that access token. (You can hardcode it in your script.)
EDIT
From https://www.dropbox.com/developers/core/start/php:
Finally, call finish to convert the authorization code into an access token.
list($accessToken, $dropboxUserId) = $webAuth->finish($authCode); print "Access Token: " . $accessToken . "\n";
The access token is all you'll need to make API requests on behalf of this user, so you should store it away for safe-keeping (even though we don't for this tutorial). By storing the access token, you won't need to go through these steps again unless the user reinstalls your app or revokes access via the Dropbox website.