Background process (sending mails)

I know this issue have been discussed a lot of times but I checked a lot of archives and documents and I couldn't find any solution. What I want to do is send a lot of mails (belive me, it's not spamming) and I thought about running the process of sending mails in background (detached from the PHP script that runs it), so the page will return immediatly and the other process will continue sending the mails in the background. I didn't find any way to run a process in the background from PHP succesfully. The manual says that using system(), if the stdout is redirected to a file, it's run in background, I tryed redirecting stdout, stderr and stdin to/from files and it still waits for the termination of the process, I tryed adding & at the end of the process line and it didn't work neither. Any idea of how I can do this (without using at, but maybe cron, I don't know how) ? Thank you.