A co-worker had a problem with using SetEnv and SetEnvIf in his Apache configuration, where the SetEnvIf didn’t seem to be working. What he was running into was the fact that those two commands are implemented in separate Apache modules which happen to run in different request phases.

In general the code was something like:

SetEnv WHICHENV prod
SetEnvIf Host eval WHICHENV=eval
SetEnvIf Host dev WHICHENV=dev

so the application can tell whether it’s on the development, evaluation, or production server. That’s how you would write the code as a program, but this example will end up always having the WHICHENV variable set to prod.

If your Apache has the server-info handler enabled (usually mapped to /server-info) you can use that to see that the SetEnv is implemented in mod_env, which runs in the Fixups phase, while SetEnvIf is implemented in mod_setenvif, which runs in the Post-Read Request and Header Parse phases, which means the SetEnvIf commands are run before the SetEnv command.

The easiest workaround is to change the SetEnv to SetEnvIf so the lines get executed in order:

SetEnvIf Host ".*" WHICHENV=prod
SetEnvIf Host eval WHICHENV=eval
SetEnvIf Host dev WHICHENV=dev