well... life would be 2 easy and boring with solution like that

the thing is that if i enter my 3th lvl domain
3th.domain.ltd
its "impossible" to get out of there
/ == 3th.domain.ltd
with absolute path i cant access anything
cant use ../ ether
once more the folder structure used:
/www_root/index.php (doc root)
/3th/some.html (so to root i tough i can get like ../www_root/ which is impossible)
so following wont help me
RewriteRule ^/?$ /root_www/index.php [L]
coz / is already /var/www/domain/3th/
as i was despaired with no solution i asked my provider to set vhost serveralias to *.domain
and i had to wrote my own php router for everything :/
so my final question would be: is it apache config related problem or just my lack of experience with mod_rewrite?
thx for replay