My work computer is a 2008 MBP Core2 Duo 2.4Ghz with 4GB of RAM and I can get by on it for PHP web development. I've actually managed to get by fine on it all the way until early 2013 without really major complains until I started doing heavy Ruby on Rails.
My 2013 MBA with CPU and RAM upgrade is so much better. For PHP, you'll be fine and wouldn't notice any slow down even with an IDE like PHPStorm or Netbeans. The new PCIe SSD really make it great to use with PHPStorm when the IDE does indexing of your projects.
I use RubyMine, PHPStorm and Xcode on mine, and they run pretty well. But sometimes at the edge of the RAM happy zone (where no swappy occur) if I've been working a few hours. A rMBP with 16GB of RAM would probably get further on this, but at the exchange of battery life. I hang around a group of IOS developers who mostly use 15" rMBP, my battery last way longer doing the same things on Xcode. They start plugging in after about 4-5 hours, I'm still at 60-70%
The most intensive task I've come up against is the IOS simulator in Xcode, old MBP used to struggle, the 2013 MBA get through it pretty well, not as fast as the quad core MBP, but good enough for everything I've needed to do.
I cheat in my development environment by offloading database operations to a home server or work server. Turn on MySQL locally when necessary, and for very small DB. If you're heavily reliant on MySQL, that will be a bottleneck, especially if you have large DB that uses innodb. I don't like to limit the InnoDB RAM usage, so will normally allocate 2GB for MySQL when it run, but that really cut down on the RAM available for IDE and background processes.
The other major bottleneck in the MBA is if I try to run unit tests in RubyMine with profiling for code coverage. Most laptops are really short here once you get to several thousand tests. That's why I'm getting a nMP for work to run testing. If you've got quite a collection of PHPUnit tests, might run into a similar bottleneck.
Other than those few limitations, I've found it to be an excellent development platform, with the understanding this is machine has a 3 years useful lifespan in how it's being used.