While testing some code to find a different way to upload files with PHP, I checked a lot of websites to get the information I need to understand cURL better. Today I found this article: Using the CURL library in PHP

With only a few examples it was clear for what usage I need cURL and where it’s better to use PHP functions. The following sentence has some important information:

“Next up is the CURLOPT_POST option. This is a very useful function, as it allows you to do POST requests, instead of GET requests, which actually means you can submit forms to other pages without having to actually fill in the form.”

That will say a web form is not posted to cCURL but to the remote server but can be posted by cURL to the next server or different location.
For the upload of files via forms is cURL not the best option, but you can upload the form to the remote server and post the uploaded data with cURL to the next location. This is useful if this location is password protected or only accepts connection from one IP address…