php - Which htaccess rules would allow for navigating a webapp as specified?

one text

I have a Web App stored in a directory htdocs/webapp. The PHP code is in an extra subdirectory on its own: htdocs/webapp/php which is where the entry point for my web app (index.php) is.

The .htaccess inside htdocs/webapp/php has the following content, especially the RewriteRules:

RewriteEngine On
RewriteBase /app/php

# page
RewriteRule    ^([a-zA-Z0-9_-]+)$  index.php?page=$1&subpage=home [QSA]
# page+subpage
RewriteRule    ^([a-zA-Z0-9_-]+)/([a-zA-Z0-9_-]+)$  index.php?page=$1&subpage=$2 [QSA]

Now, my index.php contain links and stuff. I would like to write links so that, e.g. going to the login page would read in the HTML something like:

<base href="php/"/>
...
<a href="login">Login</a>

With my current setup, I get the above link to point to webapp/php/php/login instead of webapp/php/login.

How could I fix this using a .htaccess directive? I suspect it has to do with the RewriteBase rule, but omitting it produces the same behavior and providing the absolute path:

RewriteBase /C:/Bitnami/wampstack-7.4.13-0/apache2/htdocs/webapp/php

will trigger a 403 error.

Note that I need

<base href="php/"/>

because otherwise trying following different page will cause the links to "accumulate" pages, e.g.

<a href="page2">Login</a>

will go to webapp/php/page1/page2 if the link is followed from page1, but I would like to get to webapp/php/page2.