Name: PHPSESSID Cookie
Version: 1.1
Description: Only blocks when a PHPSESSID cookie is sent by the user and it contains characters other than 0-9a-z..
Rules: 
RewriteCond %{HTTP_COOKIE} ^.*PHPSESS?ID.*$
RewriteCond %{HTTP_COOKIE} !^.*PHPSESS?ID=([0-9a-z]+);.*$
RewriteRule .* - [F,NS,L]