Welcome back! Today, we'll master what we learned about backward compatibility in practice. Prepare to apply all the knowledge to practical tasks, but first, let's look at two examples and analyze them.

Let's say that initially, we have a complex data processing class designed to operate on an array of associative arrays, applying a transformation that converts all string values within the array to uppercase. Here's the initial version using PHP:

We intend to expand this functionality, adding capabilities to filter the items based on a condition and allowing for custom transformations. The aim is to retain backward compatibility while introducing these enhancements. Here's the updated approach using optional parameters and closures:

In this evolved version, we've used optional parameters callable $condition and callable $transform for custom filtering and transformation of items. The default behavior processes all items, converting string values to uppercase, thus maintaining original functionality for existing code paths.

Imagine now that we are building a music player, and recently, market demands have grown. Now, users expect support not just for MP3 and WAV but also for FLAC files within our music player system. This development poses a unique challenge: How do we extend our music player's capabilities to embrace this new format without altering its established interface?

Let's say that we currently have a MusicPlayer class that can only play MP3 files:

Let's approach this challenge by introducing an adapter that encapsulates different formats and extensions modularly and maintainably:

This adaptation ensures that we can extend the MusicPlayer to include support for additional file formats without altering its original code. The MusicPlayerAdapter acts as a unified interface to the legacy MusicPlayer, handling formats by determining the appropriate strategy based on the file type.

Great job! You've delved into backward compatibility while learning how to utilize optional parameters, default values, and the Adapter Design Pattern in PHP. Get ready for some hands-on practice to consolidate these concepts! Remember, practice makes perfect. Happy Coding!