I'm afraid it's not just a php call. You can use any type of coding language and it doesn't have to be on a server. You can use an API Upload on any device. For example loads of mobile apps upload photos and videos to sites like Facebook, Google+ via their api.nothing a php calls to upload via script to filehost.
mean u don't have to visit website just run script on ur server and upload of files automatically starts
best example :- zoom uploader