We've been having quite a few issues with speed and server load, something which we'd never had to worry about in the past. We've been building more and more web apps and fewer and fewer simple websites.
We are also facing mod security restrictions on our webhost. Cartika (no longer recommended) are strapped down pretty tightly, but that makes sense. They also let us know right away if a website of ours is facing security attacks or if it is being scraped every day.
Apparently not all PHP is created equal and it is time to batten down the hatches.
Once we get to functionality we will have to put a full-fledged optimisation phase in the development cycle: the Code Optimization Phase.
In that phase we will specifically target PHP speed and security (as functionality will already be completely in place).
I will start by asking all Foliovision developers to read the article 40 Tips for optimizing your php Code. The top twenty or so are below.