A few weeks ago I’ve challenged to make some routing with Apache’s mod_rewrite. The task was to show http://someapp.heroku.com when user visit http://site.com/heroku. Ok, it seems to be not a problem to extend .htaccess file one RewriteRule string:

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RewriteRule ^heroku/?(.*) http://someapp.heroku.com/$1 [P,L]

But our http://site.com running Wordpress, so the new rule must be separated with conditions:

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RewriteCond %{REQUEST_URI} ^\/heroku
RewriteRule ^heroku/?(.*) http://someapp.heroku.com/$1 [P,L]

Looks nice, and it works, but Heroku returns Bad Request error when I’ve tried to visit “rewrited” page. After some derping over internet, I found, that Heroku needs request header host to be set with an app domain, someapp.heroku.com in my case. But looking forward, I’ve noted that Wordpress using request header host for it’s own purposes (e.g. link generation in admin), so I need to make another condition for request header. And here it comes - the final result:

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SetEnvIf Request_URI ^\/heroku heroku
RequestHeader set host "someapp.heroku.com" env=heroku
RewriteCond %{REQUEST_URI} ^\/heroku
RewriteRule ^heroku/?(.*) http://someapp.heroku.com/$1 [P,L]

And after that manipulations - it works like charm now!