Regardless of why you've purchased this book, the work involved to get here is certainly nontrivial, and you deserve a pat on the back. Getting here probably involved sacrificing at least some time that you'd rather have spent playing video games, attending social gatherings, interacting with your family, or just getting enough sleep.
We hope you can now appreciate some of the features and ubiquity of PHP. Go out and apply what you've learned, using baby steps if need be, but apply the lessons you've learned. Install PHP Unit. Cruise through the PEAR archives, and don't just install and run third-party PHP applications, but look at them, read them, figure out what they're doing, and fix them if need be.
Even though your skill set may be both deep and wide, you really never stop learning. Contrary to the belief that Wrox is attempting to torture you, every chapter exists for a reason and none is purely academic. PHP professionals (in the field) have created this book, and everything that was presented to you is used in a professional environment.
That also means that everything has a history. Neither this book nor PHP5 was created out of ether, so these functions and practices are worth further exploration. Entire books, even entire companies and some subindustries, support the concepts that have been presented to you. Please continue reading because some great resources are coming right up.