declaring variable in .htaccess

How to declare a variable in .htaccess file and use it there also? I tried this one but it doesn't work.

# Declare Environment
SetEnvIf base_path=""

# Prevent Directory Listing
Options -Indexes

# Default Page Redirection
# Error 400 - Bad Request
ErrorDocument 400 base_path
# Error 401 - Authorization Required
ErrorDocument 401 base_path
# Error 403 - Forbidden
ErrorDocument 403 base_path
# Error 404 - Internal Server Error
ErrorDocument 404 base_path
# Error 500 - Wrong Page
ErrorDocument 500 base_path

# Protect Sensitive Files
<Files configuration.ini>
    Order allow,deny
    Deny from allow
</Files>