Simple online course with clearly witten PHP. Teach OOP Basics.

Write a calculator in PHP that can post data to a txt file, and a MySql Database, and post /call data from an XML file. Explain the properly written code segments and make it easy to copy and paste for visitors who want to learn OOP. For example beifly and simply explain what a Class is and why it's used, same with Functions, explain what a child is and make it very clear so anyone can modify the code to suit their needs. You decide how many pages, remember this is supposed to be a simple PHP Calculator, No Java! HTML, CSS, XML, TXT only. Any form you choose will work fine.