I'm using a combination of Nginx/Passenger to run the Ruby-based pi.gadgetoid.com 24/7 on a 512Mb Raspberry Pi. This drastically outperforms the Thin/Apache setup I was using before. This is a reasonably high-traffic, public website, although I have cheated a lot by using Cloudflare ( a caching proxy basically ) to deal with most of that volume.
I'm pretty sure this configuration would sit comfortably on a 256MB Pi also, but I had trouble compiling the Passenger modules and Nginx; running into memory limitations.
I know it's not a PHP solution, and I'll likely get berated for suggesting such a tangental answer; but if you're looking for the Pi to push the bounds of your knowledge, and are doing this for some casual tinkering, I strongly suggest looking into Ruby and Sinatra. You'll have a seething hatred for PHP within a couple of weeks.
By and large you'll not have much trouble running a PHP/Apache web stack on a 256MB server. I've done this for much larger sites in a professional capacity on cloud-based hosting. It's when MySQL gets involved that things get ugly. Running a full LAMP stack on 256MB is painful to say the least, but not impossible for low traffic volumes.