mod_rewrite RewriteRule - Redirects

mod_rewrite.htaccess

<IfModule mod_rewrite.c>

    RewriteEngine on

############################################
## Define some initial variables for use in RewriteRules later

    ## Sets an env var on secure page loads for www redirects ##
    RewriteCond %{HTTPS} on [OR]
    RewriteCond %{HTTP:X-Forwarded-Proto} https [NC]
    RewriteRule ^(.*)$ - [E=SECURE:s]

    # Capture some variables from the current hostname
    # Current Sub Domain = C_SUB_D
    # Current TLD (Top Level Domain) = C_TLD
    RewriteCond %{HTTP_HOST} ([^.]+\.)*([^.]+)\.([^.]+)$
    RewriteRule ^(.*)$ - [E=C_SUB_D:%1,E=C_TLD:%3]

    # set some host and site variables
    RewriteRule ^(.*)$ - [E=HOST_FIRST:%{ENV:C_SUB_D}firstdomain.%{ENV:C_TLD}]
    RewriteRule ^(.*)$ - [E=HOST_SECOND:%{ENV:C_SUB_D}seconddomain.%{ENV:C_TLD}]
    RewriteRule ^(.*)$ - [E=SITE_FIRST:http%{ENV:SECURE}://%{ENV:C_SUB_D}%{ENV:HOST_FIRST}.%{ENV:C_TLD}]
    RewriteRule ^(.*)$ - [E=SITE_SECOND:http%{ENV:SECURE}://%{ENV:C_SUB_D}%{ENV:HOST_SECOND}.%{ENV:C_TLD}]

    # Redirects the URL to the same address at *.firstdomain.*/
    #RewriteRule ^(.*)$ http%{ENV:SECURE}://%{ENV:HOST_FIRST}/$1 [R=301,L]
    #RewriteRule ^(.*)$ %{ENV:SITE_FIRST}/$1 [R=301,L]

    # Redirects the URL to the same address at *.seconddomain.*/
    #RewriteRule ^(.*)$ http%{ENV:SECURE}://%{ENV:HOST_SECOND}/$1 [R=301,L]
    #RewriteRule ^(.*)$ %{ENV:SITE_SECOND}/$1 [R=301,L]
    
    # This works well for building redirects for site that you may have setup in 
    # multiple environments like dev, stage, prod
    # dev.firstdomain.com, firstdomain.dev, www.firstdomain.dev, stage.firstdomain.com, www.firstdomain.com
    # dev.seconddomain.com, seconddomain.dev, www.seconddomain.dev, stage.seconddomain.com, www.seconddomain.com
    
    # you can use these rules in a .htaccess file that is used by multiple domains and check
    # the current host as a condition to forward certain paths over to the other site's url
    
</IfModule>