i have the same problem.
a phpscript (binary) that i made is working fine from the commandline, but when i try to run it with exec() it does nothing, and i really needs to run it in the background but let it execute after a form post.
i'm pretty sick of it, 'couse i have been reading so much about that subject i have a headache, a really bad one.
hopefully you find a solution (cron job is not a opion really), please tell me.
thx