Non blocking functions PHP

I have a project in wich a user uploads an image through a form and the server does some thumbnails. The thumbnailing is very slow so I thought that doing the image resizing with a non-blocking function could be a good solution. I mean: the server process the form (wich have more fields) gives the "ok" feedback to the user and then calls the thumbnailing function. How can I do that?

Thanks in advance

Your best option would be to implement Gearman. It's a Job Queue system where you can implement either synchronous of asynchronous jobs. http://gearman.org/