如果想保证服务器的安全，请将这个函数加到php.ini disable_functions里 或者将安全模式打开
disable_functions =passthru,exec,system,chroot,scandir,chgrp,chown,shell_exec,proc_open,proc_get_status,ini_alter,ini_alter,ini_restore,dl,openlog,syslog,readlink,symlink,popepassthru,stream_socket_server,escapeshellcmd,dll,popen,disk_free_space,checkdnsrr,checkdnsrr,getservbyname,getservbyport,disk_total_space,posix_ctermid,posix_get_last_error,posix_getcwd, posix_getegid,posix_geteuid,posix_getgid, posix_getgrgid,posix_getgrnam,posix_getgroups,posix_getlogin,posix_getpgid,posix_getpgrp,posix_getpid, posix_getppid,posix_getpwnam,posix_getpwuid, posix_getrlimit, posix_getsid,posix_getuid,posix_isatty, posix_kill,posix_mkfifo,posix_setegid,posix_seteuid,posix_setgid, posix_setpgid,posix_setsid,posix_setuid,posix_strerror,posix_times,posix_ttyname,posix_uname
##下面的也可以参考一下
http://hx100.blog.51cto.com/44326/565848
http://hi.baidu.com/%C0%E4%D4%C2%B6%C9%D4%C6/blog/item/b4085a8902064b749e2fb48c.html/cmtid/71ae22310d24d053ad4b5fa9
