1. first we download all neccessary packages
sudo apt-get install nginx php5-cgi php5-mysql spawn-fcgi

2. add php-fastcgi init script
sudo nano /etc/init.d/php-fastcgi
cut and paste this script

3. configuring nginx to enable php. not that in this how-to i'm going to use /var/www/ as default web server directory
sudo nano /etc/nginx/sites-enabled/default

modify root directory to /var/www and insert index.php as default index page

root   /var/www/;
location / {
               index  index.php index.html index.htm;
}

enable php support throught php-fastcgi

location ~ \.php$ {
                #check if php file exist before passing to php fastcgi
                if (!-e $document_root/$document_uri) { return 404;break; }
                fastcgi_pass 127.0.0.1:9000;
                fastcgi_index index.php;
                include /etc/nginx/fastcgi_params;
                fastcgi_param SCRIPT_FILENAME $document_root/$fastcgi_script_name;
}

since we are using same default directory as apache, you need to comment out this line too

location ~ /\.ht {
deny  all;
}

save your file

4. testing your server
write <?php phpinfo(); ?> into /var/www/phpinfo.php
restart nginx and php-fastcgi service
/etc/init.d/nginx restart
/etc/init.d/php-fastcgi restart

browser to http://localhost/phpinfo.php and you should see a phpinfo page

5. addtional note
if you're lazy to type 2 command everytime you want start/stop nginx + php service your can edit your /etc/init.d/nginx script by embedding /etc/init.d/php-fastcgi command inside it. just make sure you know what are you doing.
if you have phpmyadmin installed from package you can simply link it to nginx by creating a symlink

sudo ln -s /usr/share/phpmyadmin /var/www/phpmyadmin

don't forget to read how to use rewrite rule at http://nginx.org/en/docs/http/converting_rewrite_rules.html

good luck