Some problems with mod_rewrite module

I have read a documentation of mod_rewrite module, I have started a
few tests and I have a few problems
Firstly, I couldn't set environment variable, that is to say if I set
environment variable in mod_rewrite, I can't get it in PHP
..htaccess
RewriteRule (.*) ind.php [E=A:ble]

ind.php
<?
echo'<pre>';
print_r($_ENV);
?>
I haven't the $_ENV['A']

Is this normal? or maybe I do sth wrong?

Secondly, I can't use -x in my mod_rewrite, for example:

RewriteCond path/to/the/file.ext -x

As far as I know this condition should be executed when some file have
chmod 777 or something, but I change this attribute and nothing
happens

Thirdly, I discovered something, I have some URL: http://www.example.com/sth17
and a rule in .htaccess

RewriteRule ^([a-z\.]+) index.php?$1

in index.php I print $_GET and I get one key in this array without any
value, this key is [index_php]
when I change my rule to

RewriteRule ^([a-z\.]+)([0-9]+) index.php?$1=$2

I get:

[sth] => 17

but I have found nowhere information about behaviour like this

4. I want to rewrite url like this:
http://www.subdomain.example.com/path/to/file.ext
to
http://www.example.com/subdomain/path/to/file.ext

I try with .htaccess like this
RewriteEngine On
RewriteBase /

RewriteCond %{HTTP_HOST} !^www\.zawi\.dreamhosters\.com$
RewriteCond %{HTTP_HOST} ^(www\.)?([^.]+)\.zawi\.dreamhosters\.com$
RewriteRule .* %2%{REQUEST_URI}

when I delete %{REQUEST_URI} everything is fine, and the url
http://subdomain.example.com/ is rewrited to http://example.com/subdomain/,
but when I leave it I get ERROR 500 Misconfiguration server or sth
like that

5. I found somewhere code like that

RewriteCond %{HTTP_HOST}<>%{REQUEST URI} condition_in_regexp

but I can't find nowhere what does it mean <>

thank you in advance