TL;DR: If you make www.example.com the canonical domain for your site then you should serve redirects when someone hits the naked example.com domain. If www is canonical then bookmarks and searches will always find the correct URL so ideally you'll only need to serve redirects when people type the naked address manually. As such this app should be able to handle the redirects for dozens of domains for all but the biggest sites, saving you the trouble of coding up the redirects on a per-app basis.
This is a Heroku "app" that will happily redirect "naked" domains to their www equivalent. It takes advantage of the fact that the default Heroku PHP buildpack uses Apache and supports .htaccess files with mod_rewrite rules.
This app is especially useful with S3, which can host websites these days (with a default index.html file) but can't host "naked" domains, only CNAMEs. Likewise, Heroku is happy to have your subdomains pointed at it with CNAME but gets all wriggly if you use A records with IP addresses. In most web servers/frameworks it is trivial to redirect if the host isn't canonical but using this app I can configure it all in one place and handle cases such as S3 that don't support naked domains at all.
Do:
git clone git@github.com:RandomEtc/heroku-www-redirect.git
cd heroku-www-redirect
heroku create your-app-name
heroku domains:add example.com
git push heroku master
You can add as many domains as you like, so long as they don't start with www.
Then add Heroku's IP addresses as A records in your DNS settings panel. At the time of writing these are:
75.101.163.44
75.101.145.87
174.129.212.2
These are unlikely to change but given the above wriggling you should check the Devcenter to confirm.
Single process Heroku apps are free at the time of writing but might not be forever. If they don't get much traffic they get spun down so the first request that wakes them up can be quite slow.
I haven't tested this with https, or even thought about it except while typing these words.
There's a free service offered by wwwizer that does the same thing. Just point your domain at their IP and trust them.