What causes Apache Environment Variable set using mod_rewrite to have "redirect_" appended to name?

Question

I am trying to set Apache environment variables (for use in PHP) with the [E=VAR:VAL] flag on RewriteRule rules in an .htaccess file.

I have already discovered the variables are accessed in PHP as server variables $_SERVER rather than $_ENV (which makes a certain amount of sense). However, my problem is for some rules the [E=VAR:VAL] flag works as expected and I end up with a variable $_SERVER['VAR'] but for other rules I end with a variable $_SERVER['REDIRECT_VAR'] or $_SERVER['REDIRECT_REDIRECT_VAR'], etc

A. What causes an Environment Variable set in Apache using the [E=VAR:VAL] flag to get renamed by having "REDIRECT_" appended to the front of the variable name?

B. What can I do to make sure I end up with an Environment Variable with an unchanged name so I can access it in PHP as $SERVER['VAR'] without having to resort to checking for variations of the variable name having one of more instances of "REDIRECT" appended to the front of it?

Partial solution found. Adding the following to the start of the rewrite rules recreates the original ENV:VAR on each redirect (as well as leaving the REDIRECT_VAR versions there) if they're needed:

RewriteCond %{ENV:REDIRECT_VAR} !^$
RewriteRule .* - [E=VAR:%{ENV:REDIRECT_VAR}]

Answer 1

I haven't tested this at all and I know it doesn't address points A or B, but there is some description of this issue in the comments in PHP documentation and some possible solutions for accessing these variables using $_SERVER['VAR']:

http://www.php.net/manual/en/reserved.variables.php#79811

EDIT - some more responses to the question offered:

A: The environment variables are renamed by Apache if they are involved in a redirect. For example, if you have the following rule:

RewriteRule ^index.php - [E=VAR1:'hello',E=VAR2:'world']

Then you may access VAR1 and VAR2 using $_SERVER['VAR1'] and $_SERVER['VAR2']. However, if you redirect the page like so:

RewriteRule ^index.php index2.php [E=VAR1:'hello',E=VAR2:'world']

Then you must use $_SERVER['REDIRECT_VAR1'], etc.

B: The best way to overcome this issue is to process the variables that you're interested in using PHP. Create a function that runs through the $_SERVER array and finds the items that you need. You might even use a function like this:

function myGetEnv($key) {
    $prefix = "REDIRECT_";
    if(array_key_exists($key, $_SERVER))
        return $_SERVER[$key];
    foreach($_SERVER as $k=>$v) {
        if(substr($k, 0, strlen($prefix)) == $prefix) {
            if(substr($k, -(strlen($key))) == $key)
                return $v;
        }
    }
    return null;
}