running commands sequentially in background with exec($commands_in_background)

I want to execute a couple of linux commands from a php script 'sequentially in background'. I want to run them in background so the php script doesn't hang while the linux commands are running. example: first i create a lock file with php $lockfile_path : /home/web/...blablabla.../test.tar.bz2_Lock.txt the I execute the following command from my php script: exec("du -a ~/ 2>&1 > /home/web/...blablabla.../backupFSlog.txt ; sleep 10 >/dev/null 1>/dev/null 2>/dev/null ; rm -f $lockfile_path

/dev/null 1>/dev/null 2>/dev/null &);

problem: the php script hangs for 10 seconds (sleep), what is not what I want I've read a lot of sugestions but I can't get it to work. When I execute the commands one by one, it works in background, but when I want to run them 'sequentially in background', the php script hangs They need to run sequentially because the $lockfile_path may only be deleted when all the other commands are executed Some advice please? -- http://www.dasdan.be