If you are chrooting php into enviornment that doesn't have /bin/sh you can't execute any command! Even when you call
mail() and it calls sendmail ... well actually sendmail NEVER gets called because /bin/sh is not in the chroot!
SO in conclusion: YOU MUST HAVE /bin/sh TO EXECUTE SOMETHING!!!
VERY IMPORTNAT!
I've lost few days while find this, hope it helps someone!!!