Since my last laptop was not too powerful and Eclipse would take forever to load, I got used to Notepad++ for all small to medium sized projects. Notepad++ offered all the necessary features for this type of work, including syntax highlighting, function list and multiple tabs. On top of that Notepad++ was extremely fast and had a pretty interface.
My new laptop loads applications many times faster thanks to an SSD I have installed in place of regular HD and has 4GB of RAM which is enough to run Eclipse without any issues. So, the choice was obvious and I went with Eclipse. Eclipse does take a few seconds to load, compared to couple of seconds for most applications, but I can live with that.
Just for the sake of exploring, I have tried out many editors on Ubuntu and in my opinion the best two for (PHP) code editing are: Gedit and Geany. Bare-bones Gedit is next to useless for programming, but installing a few plugins and enabling some features makes it a quite a bit of an SDK.
Geany is purpose-built for writing code and it is very responsive and has a extremely nice interface. Definitely the best light code editor for Ubuntu. I have also tried a few others, but they were either not suited for programming, looked ugly or lacked necessary features.
So, to sum it up: if you have a powerful machine, go with an industry standard Eclipse, otherwise install Geany or use pre-installed Gedit, but keep in mind that Gedit will need customization to be more or less useful for writing code.