From LedHed's Wiki
If you want to enable lighttpd to use PHP then you need to turn on fastcgi.
1. Edit the /usr/local/etc/lighttpd/conf.d/fastcgi.conf and add the following:
fastcgi.server = ( ".php" => ( "localhost" => ( "socket" => "/var/run/lighttpd/sockets/php-fastcgi.socket", "bin-path" => "/usr/local/bin/php-cgi" ) ) )
2. Uncomment the following line in /usr/local/etc/lighttpd/modules.conf
## ## FastCGI (mod_fastcgi) ## include "conf.d/fastcgi.conf"
3. Restart the lighttpd service
service lighttpd restart