.htaccess Add a parameter to query string if using certain subdomain - apache

Question

I would like to add a fixed parameter to a URL if a certain sub domain is used, but otherwise leave the URL untouched.
For example:
http://subdomain.domain.com?foo=bar
would rewrite to
http://subdomain.domain.com?foo=bar&isMySub=true
but http://domain.com?foo=bar or http://othersubdomain.domain.com?foo=bar would remain untouched.
Ive looked around on here and so far all Ive been able to ascertain is that it will likely require the [QSA] flag so as to leave the rest of the query string intact.
This get me close, but some links and images break so it has to be messing up my url or query in ways I dont want.
RewriteCond %{HTTP_HOST} ^subdomain\.domain\.com(&|$)
RewriteRule .* index.php?foo=bar [QSA]

Answer 1

You can exclude requests for certain files using the %{REQUEST_URI} server variable and checking the extension:
RewriteCond %{REQUEST_URI} !\.(css|js|jpe?g|gif|png)$
RewriteCond %{HTTP_HOST} ^subdomain\.domain\.com
RewriteRule .* index.php?isMySub=true [QSA]
The [QSA] flag will pass the existing query string, such as ?foo=bar, so you don't have to.
Alternative rules for the above ruleset:
Send to index.php and add requested resource as the query variable req:
RewriteRule ^(.*)$ index.php?req=$1&isMySub=true [QSA]
The parenthesis () in the rule pattern capture the matching value into a variable. First set of parens goes to $1, second to $2, etc.
Or, send to same resource as is requested but with the added query variable.
RewriteRule ^(.*)$ $1?isMySub=true [QSA]