I have a web page with a form. I want the user to upload file to a Dropbox folder and get the download link. Also I don't want the form to ask the user for authentication in order to upload the file.
How can I do this?
I registered an App for this purpose and read the Api documentation, but it seems to me to complicated and can't even start.
On
"Also I don't want the form to ask the user for authentication in order to upload the file."
You certainly can't upload files into a user's Dropbox without that user's permission.
The Saver is the closest to what you want in terms of allowing a user to save a file to Dropbox without having to authorize your app, but you won't get back a link to the file.
The Chooser also does something like this, which is to give the user the option of uploading a file to Dropbox before then choosing it. You could instruct the user to upload a file to Dropbox in the Chooser and then select it. After that, your app can get back a link to the uploaded file. You can try this out yourself in the Chooser demo and see if it meets your needs.
Dropbox website has a pretty good rundown of what you're trying to do in php: https://www.dropbox.com/developers/core/start/php
You can use an html form on your website that POSTs to a php page.
There should be plenty of available guides on the internet for setting up a form that saves the file on to your server using php/forms. You then just need to redirect the file to your Dropbox, using their API.