Dear Friends
if you have an error like this:
[Sat Apr 25 11:02:55.393957 2013] [:error] [pid 3343] [client 192.168.1.1:57325] PHP Warning: opendir(/var/ossec): failed to open dir: No such file or directory in /opt/ossec/lib/os_lib_handle.php on line 94, referer: http://192.168.1.20/ossec
solution:
find your ossec_conf.php file and edit:
change the ossec directory
enjoy