How to redirect different sub domain requests to different port

I will assume that you have your own reason for wanting the two sites (www and blog) to run on different ports - and in different processes. If this is not what you intended, e.g. you did not want to have two distinct processes, then having different ports may not be what you intended either: use VirtualHost instead, to co-host the two domains within the same apache+php instance on port 80. Otherwise, read on.

Assuming that you have your two apache+php processes listening on localhost:82 and localhost:83 respectively, bring up a third, apache-only process to act as a reverse proxy. Have the reverse proxy apache instance listen for requests coming on port 80 from the internet, with two virtual host definitions. The first virtual host definition, www, would forward requests to localhost:82, whereas the second virtual host definition, blog, would forward requests to locahost:83, e.g.:

NameVirtualHost *:80

# www
<VirtualHost *:80>
  ServerName www.myexample.com
  ProxyPass               /       http://localhost:82/
  ProxyPassReverse        /       http://localhost:82/
</VirtualHost>

# blog
<VirtualHost *:80>
  ServerName blog.myexample.com
  ProxyPass               /       http://localhost:83/
  ProxyPassReverse        /       http://localhost:83/
</VirtualHost>

I use proxy for this type of things.

In my example, I have apache 1.3 running on port 80, but I needed svn repository to run on apache 2.2, and I didn't want to type :82 on the end of the domain every time. So I made proxy redirection on apache 1.3 (port 80):

<VirtualHost *:80>
  ServerName svn.mydomain.com
  ServerAlias svn
  ServerAdmin [email protected]

  <IfModule mod_proxy.c>
    ProxyPass / http://svn:82/
  </IfModule>
</VirtualHost>