nginx: nginx version: nginx/1.0.1
nginx: built by gcc 4.2.1 20070719 [FreeBSD]
nginx: configure arguments: --prefix=/usr/local/etc/nginx --with-cc-opt='-I /usr/local/include' --with-ld-opt='-L /usr/local/lib' --conf-path=/usr/local/etc/nginx/nginx.conf --sbin-path=/usr/local/sbin/nginx --pid-path=/var/run/nginx.pid --error-log-path=/var/log/nginx.log --user=www --group=www --with-debug --http-client-body-temp-path=/var/tmp/nginx/client_body_temp --http-fastcgi-temp-path=/var/tmp/nginx/fastcgi_temp --http-proxy-temp-path=/var/tmp/nginx/proxy_temp --http-scgi-temp-path=/var/tmp/nginx/scgi_temp --http-uwsgi-temp-path=/var/tmp/nginx/uwsgi_temp --http-log-path=/var/log/nginx-access.log --with-http_gzip_static_module --with-http_stub_status_module --with-pcre

I decrease the fastcgi buffer size and increase the amount of buffers:
fastcgi_buffer_size 64k;
fastcgi_buffers 64 64k;

but a 'too big header' error still apears:

[debug] upstream split a header line in FastCGI records
[error] upstream sent too big header while reading response header from upstream

btw,php is working with Zend-Optimizer,whether this condition can make the headers bigger than its real size?