PHP exec in background using & is not working

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http://stackoverflow.com – I am using this code on Ubuntu 13.04, $cmd = "sleep 20 &> /dev/null &"; exec($cmd, $output); Although it actually sits there for 20 seconds and waits :/ usually it works fine when using & to send a process to the background, but on this machine php just won't do it :/ What could be causing this?? (HowTos)