ProxyPass: Redirect() reveals hidden URL

[spotlight2001]

I have a virtual url say:
wwWebmasterWorldebserver.com/redirect/

which points to another server:
(from apache config)
ProxyPass /redirect [anotherserver:9999...]
ProxyPassReverse /redirect [anotherserver:9999...]

On surfing 08/15 hyperlinks work perfectly. But there are also FORMs. The form receiver (<form action=blabla>) uses a PHP REDIRECT() ... after the redirect the url is no longer
wwWebmasterWorldebserver.com/redirect/blabla
but:
[anotherserver:9999...]

Question: How to maintain the masqueraded virtual url?