Apache 1.3.24 config file:

./configure --prefix=/opt/apache \
            --enable-module=userdir \
            --enable-module=rewrite \
            --enable-module=info \
            --activate-module=src/modules/php4/libphp4.a

and php 4.2.0:

./configure \
        --prefix=/opt/php \
        --with-openssl=/usr/local/ssl \
        --with-imap=/usr/src/local/imap-2001a \
        --with-gettext \
        --with-gettext=/opt/imp \
        --with-xml \
        --with-expat-dir=/opt/imp \
        --with-imap-ssl \
        --with-apache=../apache_1.3.23 \
        --with-mysql=/usr/local/mysql \
        --with-ldap=/opt/openldap \
        --with-db3=/opt/bdb \
        --enable-track-vars 

After updating with the above and restarting the apache server (for IMP 3.0). The server proxies requests for SSL (from httpd.conf):

    Alias /ECN/mailman/archives /usr/local/mailman/archives/public
    ScriptAlias /ECN/mailman/ "/usr/local/mailman/cgi-bin/"

RewriteEngine On
RewriteRule ^/ECN/webmail$ https://Engineering.Purdue.edu/ECN/webmail/horde/imp/
 [R]
RewriteRule ^/ECN/webmail/$ https://Engineering.Purdue.edu/ECN/webmail/horde/imp
/ [R]

RewriteRule ^/ECN/mailman$ https://Engineering.Purdue.edu/ECN/mailman/listinfo [
R]
RewriteRule ^/ECN/mailman/$ https://Engineering.Purdue.edu/ECN/mailman/listinfo
[R]

After updating to PHP 4.2.0 we started getting proxy errors for non-Solaris web browsers only:

> Proxy Error
>
> The proxy server received an invalid response from an upstream server.
>
> The proxy server could not handle the request GET /ECN/webmail/horde/imp/.
>
> Reason: Document contains no data

The httpsd server receiving the request was getting the following error:

The proxy server received an invalid response from an upstream server.

After reverting everything back to 4.1.2 it started working again. I probably don't have enough data for you to do anything about it but I thought I would submit something about it.
--mike