How does PHP File Size Affect Performance?

Simple Question: Is it better to split code between a bunch of smaller files or put it all in one big file.

Background:
I have a Website application that has about 50,000 lines of code to run the whole thing (event registration in a shopping cart format).

This is an ongoing project so most of the deployment files are not compressed and they have a lot of comments and notations. So at this time yet there still is a lot of code bloat.

I have around 100 different functions that I've written to perform various tasks.  I am wondering what option is best for bringing the best performance. There are four main tasks: 1) Login and Authentication; 2) Registration; 3) Housing; 4) Payment.  Each of these tasks is handled by a parent php files (e.g. auth.php, reg.php, house.php, pay.php) that calls in a bunch of other files as needed (load.php, navbars.php, etc.)

There are about 30 of the 100 functions that are used by all four, but then the remaining 70 are used by only one of the processes.  

Restated Question:
Will my site perform better if I put all 100 functions in one big file (of 25,000 lines) or split them up based on use? Will the one big file get cached and perform better or will it work better to split it all up. So which of these options?  

1) Put all 100 functions in one big file.
2) Put the 30 common functions in one file and then split the others into separate files based on the parent script that calls them.
3) Put the functions at the front of the parent file and make the parent file bigger.

What I'm looking for here is advice regarding performance.

Thanks.