.htaccess error 500 in production, not in development. Environment vars - apache

Question

I have a site that loads fine in my local development environment, but bombs out with HTTP Error 500: Internal server error in production. I don't have any access to apache error_log as it is on a shared hosting environment.
I think the issue is with setting of apache environment variables in .htaccess - possibly causing some kind of infinite loop? I require these as they are picked up by PyroCMS and used to determine environment specific settings such as DB config.
<IfModule mod_rewrite.c>
# Make sure directory listing is disabled
Options +FollowSymLinks -Indexes
# disable the Apache MultiViews directive if it is enabled on the server. It plays havoc with URL rewriting
Options -MultiViews
RewriteEngine on
# Automatically determine and set the PYRO_ENV variable
RewriteCond %{HTTP_HOST} ^(localhost|local.mydomain.com|mydomain|mydomain.local)$
RewriteRule (.*) $1 [E=PYRO_ENV:development,L]
RewriteCond %{HTTP_HOST} ^staging.mydomain.com$
RewriteRule (.*) $1 [E=PYRO_ENV:staging,L]
RewriteCond %{HTTP_HOST} ^www.mydomain.com$
RewriteRule (.*) $1 [E=PYRO_ENV:production,L]
# Rewrite "domain.com -> www.domain.com"
RewriteCond %{HTTPS} !=on
RewriteCond %{HTTP_HOST} !^www\..+$ [NC]
RewriteCond %{HTTP_HOST} (.+)$ [NC]
RewriteCond %{HTTP_HOST} ^mydomain.com$ [NC]
RewriteRule ^(.*)$ http://www.mydomain.com/$1 [R=301,E=PYRO_ENV:production,L]
# Keep people out of codeigniter directory and Git/Mercurial data
RedirectMatch 403 ^/.*/(system/cms/cache|system/codeigniter|system/cms/config|system/cms/logs|\.git|\.hg|db).*$
# 301 permanent redirects for old web site pages
RewriteRule ^some/old/path$ /some/new/path [R=301,L]
RewriteRule ^some/other/old/path$ /some/other/new/path [R=301,L]
# Send request via index.php (again, not if its a real file or folder)
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
<IfModule mod_php5.c>
RewriteRule ^(.*)$ index.php/$1 [L]
</IfModule>
<IfModule !mod_php5.c>
RewriteRule ^(.*)$ index.php?/$1 [L]
</IfModule>
</IfModule>
If I comment out the following lines:
RewriteCond %{HTTP_HOST} ^www.mydomain.com$
RewriteRule (.*) $1 [E=PYRO_ENV:production,L]
Then the error 500 disappears and instead I get a DB connection error (as PyroCMS cannot detect the environment).
Usually, I would set the environment variables directly into the vhost config using the SetEnv directive. However, as this particular site is on shared hosting, I have no access to these.
How can I resolve this issue?
On another point, I have sections below that provide 301 redirects for old website pages to new ones (this is a rebuild of an old website). My gut is telling me that even if I resolve this issue, the Environment variable that is set near the top of the .htaccess file will be lost when another RewriteRule is matched further down the .htaccess file. Am I correct in this thinking?

Answer 1

The solution involved removing the L flag from the RewriteRule's on that set the environment. The L flag was stopping further rules from being processed and invoking an infinite loop. By removing the L flag, it means that the environment is set and applies to all further redirects. This also answers my aside question about being able to persist the ENV after another 301 redirect.
# Automatically determine and set the PYRO_ENV variable
# We DON'T use the L flag on RewriteRule here as we want it to persist through redirects.
RewriteCond %{HTTP_HOST} ^(localhost|local.mydomain.com|mydomain|mydomain.local)$
RewriteRule (.*) $1 [E=PYRO_ENV:development]
RewriteCond %{HTTP_HOST} ^staging.mydomain.com$
RewriteRule (.*) $1 [E=PYRO_ENV:staging]
RewriteCond %{HTTP_HOST} ^www.mydomain.com$
RewriteRule (.*) $1 [E=PYRO_ENV:production]