PHP

Make the following changes to /etc/php.ini:

# Do not expose PHP error messages to external users
display_errors = Off

# Enable safe mode
safe_mode = On

# Only allow access to executables in isolated directory
safe_mode_exec_dir = php-required-executables-path

# Limit external access to PHP environment
safe_mode_allowed_env_vars = PHP_

# Restrict PHP information leakage
expose_php = Off

# Log all errors
log_errors = On

# Do not register globals for input data
register_globals = Off

Install lighttpd and php

1. Read http://gentoo-wiki.com/HOWTO_Lighttpd and decide which USE flags you will need. Write those flags into /etc/portage/package.use. Since I plan to use fastcgi and php, I do have at least these two flags set.
2. Check php installation. If php was installed but cgi USE flag was off, you have to reinstall php with "USE=cgi".
3. Now install lighttpd:
   # emerge -v lighttpd
4. Among the few opcode cache for php, xcache is quite stable on lighttpd. So install that:
   

Adjust the parameters

Having installed APC for PHP does not automatically improve the performance. You have to adjust the parameters. At the very least, change these values:

apc.ttl="7200"
apc.user_ttl="7200"
apc.shm_segments="3"
apc.shm_size="90"

Setting apc.ttl and apc.user_ttl to none-zero can ensure that the cached php code gets refreshed at the given number of seconds without filling up the memory with stale entries.