I'm not sure how many people are running Apache on non-server Mac OS X, but if you are and upgrade from Yosemite to El Capitan, you'll get some breakage. The breakage won't be as bad as upgrading from 10.10.4 to 10.10.5 was.
Apache generally will still work (as in the server will server up files that can be visited in a web browser). You'll just have to edit the httpd.conf file to get some other things working.
Paste this command into the terminal to edit the httpd.conf file:
While you're in there, if you're using SSL, you may also want to uncomment these three lines (you may have to search for them using Control-W):
- Include /private/etc/apache2/extra/httpd-ssl.conf
- LoadModule ssl_module libexec/apache2/mod_ssl.so
- LoadModule socache_shmcb_module libexec/apache2/mod_socache_shmcb.so
Once you're done, run a