David Sklar has written a book on a subject that most other PHP authors have ignored, and which many PHP programmers have failed to consider: how using pre-made, existing libraries effectively can save you hours of writing (and re-writing) PHP code.
There are many external libraries available to PHP, some under the PEAR brand ( [...] ) and others by individual authors or companies.
If you would like to automate the creation, processing and validatin of form processing, there's HTML_QuickForm. If you'd like to implement a templating system to separate presentation logic from other programming logic, there's Smarty. If you would like to setup a lightweight, yet completely usable and effective web services platform, there's the PEAR XML-RPC library.
In many of these cases, the online documentation is too complicated or technical (e.g. HTML_QuickForm) or too skimpy (PEAR XML-RPC). Enter David Sklar's book!
David uses concise, clear language with plenty of examples to explain how a certain library works. He goes through it step-by-step, first with a paragraph of descriptive text, then with a snippet of code elucidating what was previously mentioned.
He manages to cover just enough ground to enable you to grasp how to use a certain library without boring you to tears with extraneous, non-essential information.
If you would like to avoid re-inventing the wheel and the introduction of bugs (that almost always follows the insertion of new, untested code), I suggest you procure a copy of Essential PHP Tools today, and read it cover-to-cover!