PHP programming strategy; lots of little include files, or a few big ones?

I have a flexible program, which can do many different things according to the type of data it is fed. Ideally the flexibility is achieved by calling different functions, though when the functionality is ill-defined I sometimes just include blocks of code. Ideally, from the point of program maintenance, each module should not be too long -- preferably just a page or so. This doesn't raise problems in a compiled language, but in an interpreted language like PHP the programmer must decide whether to lump a whole lot of functions into a single large include file, or to include lots of little files as the particular functions are needed. The first case can lead to memory bloat, as there are likely to be a lot of unused functions in memory on any given pass, whereas the second case may require lots of little files to be loaded. Are there likely to be significant performance costs for either approach, and what are your feelings about the relative virtues of the two approaches?