Welcome! Today, we will delve into managing a document's editing history using stacks in PHP. Imagine building a text editor; you would need to handle actions like adding text, undoing changes, and redoing them. We will see how these features can be efficiently implemented using stacks, which are represented by arrays in PHP. PHP's built-in functions like array_push and array_pop allow us to effortlessly manage these stacks. By the end of this lesson, you will possess an in-depth understanding of applying stacks in practical scenarios using PHP.

Before starting the coding portion, let's dissect the methods we will implement. These methods will manage a document's edit history, allowing us to apply changes, undo them, and redo them effectively.

• function applyChange($change): This method applies a change to the document. The change, represented as a string, is stored in a way that allows us to remember the order of applied changes. Any previously undone changes are discarded.

• function undo(): This method undoes the most recent change and allows us to store it for a possible redo. It returns the change that was undone, or null if there are no changes available to undo.

• function redo(): This method redoes the most recent undone change, making it active again. It returns the change that was redone, or if there are no changes available to redo.