Some of you may wonder what I really do at Yahoo since I joined in Sept 2007. Well, here is the brief description. I work in the Core Software Infrastructure Group at Yahoo. We are basically the people who provide the standard Yahoo LAMP serving stack, minus the L (which is another team,entirely). So maybe, you can consider us the AMP [Apache, MySQL and PHP] along with a few other core components that no one ever sees :/ The executive summary is that I don’t touch any user facing code; rather, I handle the software that talks from your web browser to our servers. It’s behind the scenes code that (among other software) makes Y! so fast and flexible.
Generalizing a bit: Venu handles the MySQL problems. Brian and Ryan handle Apache.
I am now being handed PHP. Although with Rasmus, Sara and Brian at Yahoo, it makes this an easy task 
Maybe I can start adding some interesting tidbits about PHP internals once I understand them myself.
We also have some cool libraries and apache modules that make other developers lives easier. For example, a standard library to generate and validate our own B Cookie. Libraries for generating Internal Yahoo HTTP headers. Along with some cool security modules in Apache whose details are probably a little secret for a public forum. We also support a modified stunnel for tunneling SSL traffic. Finally, our group (although not me) supports an amazingly bad ass piece of software originally from Inktomi called trafficserver. It is a super high performance HTTP proxy that blows the doors off mod_proxy, varnish or Squid.
All-in-all, it’s actually a very interesting group. One I am happy to be a part of.