Multisite

# 1) Enable Multisite in wp-config.php (above the require_once... line)
define('WP_ALLOW_MULTISITE', true);

# 2) Choose subdomains vs subdirectories DURING setup
#    - Subdirectories (example.com/site1):  works on any install
#    - Subdomains    (site1.example.com):   requires wildcard DNS + Apache/nginx rewrite

# 3) Add network constants AFTER 'Tools -> Network Setup' gives you the snippet
define('MULTISITE', true);
define('SUBDOMAIN_INSTALL', false);                 // true for subdomains
define('DOMAIN_CURRENT_SITE', 'example.com');
define('PATH_CURRENT_SITE', '/');
define('SITE_ID_CURRENT_SITE', 1);
define('BLOG_ID_CURRENT_SITE', 1);

# 4) .htaccess rules — replace the standard WP block with the Network Setup output

# 5) Useful WP-CLI commands once you are multisite
wp site list --fields=blog_id,url,name
wp site create --slug=stores-sydney --title='Sydney store'
wp site delete 4 --yes
wp super-admin add alice                   # grant network-level admin
wp option get siteurl --url=stores-sydney.example.com

# 6) Network-activate a plugin (works on every sub-site)
wp plugin activate yoast-seo --network

# Activate per-site for a specific subsite
wp plugin activate woocommerce --url=stores-melbourne.example.com

# 7) Theme strategy
#    - Use ONE theme, with theme.json palettes per site, when sites should feel unified
#    - Use child themes for per-site overrides without forking
#    - Network admins enable themes for the network; site admins choose from those

# 8) User strategy
#    - Roles are PER SITE (a user can be Editor of A, Author of B, none of C)
#    - 'Super Admin' is a NETWORK role with full power; grant sparingly

# 9) Data + isolation
#    - Each subsite gets wp_<ID>_posts, wp_<ID>_options, etc.
#    - wp_users and wp_usermeta are SHARED — same email = same person
#    - Backups: pg_dump / mysqldump the whole DB; per-site export with WP-CLI
wp db export shop-network-$(date +%F).sql

# 10) Performance considerations
#    - One persistent object cache (Redis / Memcached) is shared across sites
#    - Use a CDN that respects the Host header; subsite URLs must hit subsite caches
#    - Disable autoloading on rarely-read options on busy sites
#    - Don't enable 100 plugins network-wide; each loads on every site request

# 11) When NOT to use Multisite
#    - Sites need wildly different plugin sets and update cadences
#    - You need separate DB instances for data sovereignty
#    - You want per-site infrastructure isolation (memory limits, PHP versions)
#    -> Just run separate installs. Multisite reduces ops only when the sites
#      share enough to make the trade worth it.

# Run many sites on one install.
# Add: define('WP_ALLOW_MULTISITE', true); to wp-config.php