What is the difference between ${var} and %{var} in httpd.conf?
Pretty much everything. There's no hard-and-fast rule that says "$ does x, % does y", because they do different things depending on where and how they're used.
Will ${VAR} be used everywhere in httpd.conf, except in mod_rewrite directive's
No - ${VAR} is applied as a replacement in every location throughout the config, even in a mod_rewrite related line. These can only be defined at startup using the -D option passed to the httpd binary, or the Define directive.
This can be done because those variables must not contain a : character, whereas the RewriteMap usage must contain a : character. This allows for the $ to be used in extremely different ways with no conflict - the parser sees the : and knows that it's not supposed to be replacing that.
Would a variable set in httpd.conf using SetEnvIf Directive, for use in same httpd.conf, be used as ${var} except when the variable is used with mod_rewrite directive's, where the variable would be used as %{var}?
No. Environment variables are not the same as variables set via -D or Define - they're a completely separate per-request context (those are the ones you're setting with SetEnvIf), that only certain directives will use. They're specifically mentioned where supported - in mod_rewrite they're %{ENV:variable}, in logs they're %{variable}e, and a lot of directives have a logic check that looks like env=variable.
So, basically; it's a confusing mess - a testament to the web server's history over the past couple decades. Probably the main thing that's confusing you is that mod_rewrite's syntax doesn't extend to any other part of the config in any way; it's all different. The main point to take from all this is that there isn't a governing 'rule' to make all of this make sense - a $ in one spot means something completely different from a $ in another spot.
A few handy reference points:
- If you're looking to set a variable statically when Apache starts, use
Define and ${VAR} to replace once, on Apache's startup. This is mostly useful for when you want to do something like share config files between a development and production environment - but it's not to be confused with the use of the $ from RewriteMap, which always contains a :.
- If you're looking to work with variables in specific requests, then environment variables are what you're after - but you cannot make assumptions about the syntax that you'll use to get at them when used with a specific directive. You'll need to check the documentation for that directive.
- When mod_rewrite is in question, throw out everything you thought you knew about variables and their syntax. It has its own set of variables in
%{VAR}, you can get at your 'normal' environment variables with %{ENV:VAR}, and you can set environment variables with [E=VAR].
Try the next config:
LoadModule php5_module libexec/apache24/libphp5.so
<IfModule php5_module>
<FilesMatch "\.(php|phps|php5|phtml)$">
SetHandler php5-script
</FilesMatch>
DirectoryIndex index.php
</IfModule>
<IfModule mime_module>
...
AddType application/x-httpd-php-source .phps
AddType application/x-httpd-php .php
...
</IfModule>
I hope this help.
Best Answer
See Expressions in Apache HTTP Server.