Text tools

Hello, I was just wondering how to build a text editing tool with php. I mean a tool that can help users to format their texts in the pages. I mean something like paste the text in a text box. Select some part of the text and hit a button to, for example, bold the selected part of the text. Or align the text to left or right side of the table. Little bit like you can see in a text editor progs like MS Word. I've seen a few applications like this but I have no idea how they are made and can this be done using php. Any ideas or guidelines how to get started? Thanks Will