Jim Plush wanted to try a little experiment with perfomance in PHP - specifically dealing with the placement of the class code in an application.
Out of sheer boredom I wanted to see which was better, including one big file of classes or splitting your classes up into multiple files. I'm going to use nusoap as an example. The NuSoap package lets you download one file. nusoap.php. That one file contains 9 classes. While not only is this method nice as you only need to include one class, its also 24% faster on average than having to include 9 seperate files.
In his informal testing he found that combining the files/classes into a single large file made for better performance (as far as object usage) - of course, that also makes it harder to maintain...