I enabled mod_php3 in Apache both by using apxs, then by using the Apache ./configure --add-module approach. I followed the instructions at the Apache website for installing third party modules. Both completed without errors, but either way, when I try to start HTTPD, I get the message:

Syntax error on line 231 of /etc/httpd/httpd.conf:                           
Cannot load /usr/local/apache/libexec/mod_php3.so into server: /usr/local/apache/libexec/mod_php3.so: Undefined symbol "php3_ini"                            
/usr/local/apache/bin/apachectl start: httpd could not be started

I tried renaming my php3.ini file to php3_ini but that was just a shot in the dark.

My httpd.conf file includes the lines

LoadModule php3_module        libexec/mod_php3.so

and 

AddModule mod_php3.c

Both are at the end of their respective sections. The php3.ini file is located in /etc/httpd. I would include the php3.ini file here but it's not even being read and it's just the default configuration.

I configured PHP this way:

./configure --with-apxs=/usr/local/apache/bin/apxs \
              --with-config-file-path=/etc/httpd \
              --with-mysql \
              --with-gd \
              --with-zlib \
              --with-system-regex

Apache was originally configured:

./configure --sysconfdir=/etc/httpd \
              --datadir=/home/httpd \
              --logfiledir=/var/log/httpd \
              --enable-module=most \
              --enable-shared=max \
              --disable-rule=WANTHSREGEX

Then the php module was added:

#> apxs -c mod_php3.c
#> apxs -i -a -n php3 mod_php3.so

(completed without errors and added appropriate lines in httpd.conf)