Today I found a great PHP tutorial series called “Diving into PHP”. It is made by Jeffrey Way from nettuts and it has 15 episodes till now. It starts from scrach and slowly builds up to advanced topics that is why it is great for PHP beginers and for advanced users.

This is a very interesting tutorial if you would like to build your own calculator written in PHP. It guides you through all the steps and it is very clean written and has a good error checking. It uses if, witch and other math functions.