How to configure nginx+php (fcgi) to run every subdomain with a different user?

Im pretty new to nginx and - more for learning purpose - im trying to configure Nginx in order to run php with a different user for every subdomains.

For example, i would like to use user john for all scripts on foo.example.com and user jack for bar.example.com.

I have already created the user on my system (ubuntu server), but i dont know how to instruct nginx to use the users - and im looking for a solution tht can handle easly many users, lets say ~2000.

Looking the documentation, i dont understand if i have to spawn a php5-cgi process for every user (with a different port) and then grab them into my sites-available sites (as i said im a newbie, but this looks to me like a server-suicide), and the only 2 pages in the nginx configuration that talk about that.. is written in chinese (page1, page2), hard to translate with google translate (but, looking the codes, uses a totally different from the server-suicide way)

Any suggestion?

Update

galador's answer do the job, but im trying to build a dinamycal environment (with wildcards subdomains) that doesnt need torestart nginx/fpm for every new site, is this possible?

Edit: I just noticed your "needs to scale to ~2000 users" requirement... this might not be your best option, but could probably be easily automated with a bit of scripting.

You could use php-fpm to do something like this (fpm is part of the PHP since PHP 5.3.3. I host a couple of sites on my VPS, and use something similar.

My main php-fpm.conf looks like:

;;;;;;;;;;;;;;;;;;;;;
; FPM Configuration ;
;;;;;;;;;;;;;;;;;;;;;
include=/usr/local/etc/fpm.d/*.conf

;;;;;;;;;;;;;;;;;;
; Global Options ;
;;;;;;;;;;;;;;;;;;

[global]
; Pid file
; Default Value: none
pid = /var/run/php-fpm.pid

; Error log file
; Default Value: /var/log/php-fpm.log
error_log = /var/log/php-fpm.log

; Log level
; Possible Values: alert, error, warning, notice, debug
; Default Value: notice
;log_level = notice

; If this number of child processes exit with SIGSEGV or SIGBUS within the time
; interval set by emergency_restart_interval then FPM will restart. A value
; of '0' means 'Off'.
; Default Value: 0
;emergency_restart_threshold = 0

; Interval of time used by emergency_restart_interval to determine when 
; a graceful restart will be initiated.  This can be useful to work around
; accidental corruptions in an accelerator's shared memory.
; Available Units: s(econds), m(inutes), h(ours), or d(ays)
; Default Unit: seconds
; Default Value: 0
;emergency_restart_interval = 0

; Time limit for child processes to wait for a reaction on signals from master.
; Available units: s(econds), m(inutes), h(ours), or d(ays)
; Default Unit: seconds
; Default Value: 0
;process_control_timeout = 0

; Send FPM to background. Set to 'no' to keep FPM in foreground for debugging.
; Default Value: yes
;daemonize = yes

And then, in the fpm.d folder, I have configuration files for each site like this:

[myuser]
listen = 127.0.0.1:9000
listen.allowed_clients = 127.0.0.1
user = myuser
group = myuser

pm = dynamic
pm.max_children = 15
pm.start_servers = 3
pm.min_spare_servers = 1
pm.max_spare_servers = 5
pm.max_requests = 2000

request_slowlog_timeout = 5
slowlog = /home/myuser/tmp/logs/myuser.slow.log

php_admin_value[error_log] = /home/myuser/tmp/logs/myuser.error.log
php_admin_flag[log_errors] = on

Then, for each site, you change the user and the port in their own file, and in the nginx config, you'd have something like:

location ~ .*.php$ {
    include /usr/local/etc/nginx/fastcgi_params;
    fastcgi_pass  127.0.0.1:9000;
    fastcgi_index index.php;
    fastcgi_param  SCRIPT_FILENAME $document_root$fastcgi_script_name;
}

Changing the port in the fastcgi_pass directive.