parse speed vs require speed

I'm wondering if I should organize the functions for my project into a single large file and have only one include/require, or break the code out into separate function library files and use a number of includes/requires. With a single large function library I'm guessing PHP would parse this every time a page is called. So I could loose some overhead by separating my database functions into one file, my administration functions into another, my presentation functions into another, etc... then only including the function pages necessary - likely cutting the amount of code being parsed in half for any given page. But then I begin wondering about the speed of making a require() to each of these files. maybe three to five requires per page. Lastly, what about caching or compiling the function libraries so they're not being parsed each time? Any advice on this?

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