Php – your suggestion to write clean and well organized php code

Why reinvent the wheel? Try ELO rating system - it was developed for exactly this purpose, and is best known for being used to rate chess players, although not exclusively. And it stood the test of time, to be sure.

As for calculating the probability that player (or team) A defeats B, look under http://en.wikipedia.org/wiki/Elo_rating_system#Mathematical_details for a formula.

Here's a piece of ready-made code that implements ELO: https://github.com/Chovanec/elo-rating

It's in PHP, since you mention it specifically, but I'm sure you could find something similar for every major language.

Note that I have not tested it and I'm in no way affiliated with the author.

There are two problems in this.

Firstly storing results (in database or in cache etc.) so that the calculations does not have to be remade every time a page is loaded.

Secondly you need a mechanism to redo all calculations, you could have found a flaw in an old calculation or old data is updated/purged or new calculations are added.

The best way to do this is to add tables in the database that contains the calculations up and until a given time (eg. yesterday at midnight). As new calculations are added you'll need to update those tables or add new tables.

Whenever you need to you can empty the tables and run a script that does the calculations. This could very well be a script run automatically each night.

But you will also need to be able to clear the tables and run the script manually.

One important note is that the statistics tables are not supposed to contain permanent data. The data only needs to be easily regenerated.

The idea that old data never changes is a dangerous assumption. You can have data sources that no longer are considered reliable. There can be many reasons for old data to become obsolete. Designing a system to rely on old data never changing is designing for failure. You should design for not having to rely on old data.