Head First Design Patterns for PHP 5
Version 1.0.0, released 7/28/2009
Table of Contents
- How to Run the Command Line Examples
- How to Run the MVC Example
- Special Notes by Pattern
First let me say that Head First Design Patterns is a fantastic book serving as a solid and easy to understand introduction into Design Patterns. I am a much more in tune with the appropriate use of patterns because of the efforts of its authors. To help further my understanding of the patterns, Java, and PHP (I started my career in PHP but also have jumped into Java), I decided to follow the excellent examples set by Mark McFadden (C# port) and Glenn Puchtel (C++ port) and port the examples to PHP.
I know what you may be thinking: PHP is a Web-oriented language and doesn't really hold its own against the object oriented power of Java. Not so! While PHP is aimed mostly at Web development, its object oriented feature set as of PHP 5 is quite comparable to Java's. In fact, many of the examples ported directly, with only minor syntax changes.
Only about 6 examples couldn't be ported easily to PHP, or didn't make a lot of sense to port. For example, patterns that made extensive use of Java's GUI classes were skipped. Also, an applet port was skipped because the concept it was trying to demonstrate (hooks) was already covered by another example.
All in all, I enjoyed porting the code and hope that it helps others to understand the differences (and similarities) between PHP and Java.
If you have any questions or feedback, I'd love to hear from you!
- PHP 5.2 or better
- Sockets extension enabled for running Proxy Pattern
- Windows (tested with Server 2000), Linux (tested with Ubuntu 8.10), Mac OS X (tested with 10.5.7)
- Web server to run MVC example (tested with Apache HTTP Server 2 and IIS 5)
This may be a touchy area because there's always a lot of subjectivity when porting from one language to another. My goal was simple: do as little rewriting of the code as possible to preserve the original intent of the example and pattern. Having said that, sometimes I leave a lot of code in Java form, and sometimes I rewrote code using more native PHP concepts. The mix hopefully combines the best of both worlds: getting the pattern across but also demonstrating best practices using PHP in an elegant way.
What follows are notes on some of my decisions and approaches to porting the code.
Since PHP has no notion of packages, I used a class loader in PHP to mimic Java packages and reduce the amount of "require"/"require_once" calls. If PHP finds the referenced class in the current working directory, it loads it automatically. Classes that fall outside of the current working directory however are explicitly required much like how a Java import statement would be used. This to some extent allows all of the classes in one directory to behave like a Java package. In PHP, they are merely a group of related classes.
In PHP, if no visibility is given for a member or method inside a class, it defaults to public. For compatibility with Java's default visibility, Java members and methods which have no explicit visibility were ported as protected. Java's default visibility is somewhat like a cross between public and private access. It's public for all classes in the same package, but private to everything else. In PHP, protected seemed like a good compromise, and only cases where members or methods needed to be public was the visibility changed in order for the examples to run.
To run an actual class, PHP simulates the main() method, and separate run[className].php scripts are used to simulate the JVM calling this method on the classes. Command line arguments are also passed in and manipulated to mimic Java's String arguments. This helped reduce how much code had to be rewritten and how much relearning is needed to understand how to run these examples, if you already know how to run the Java examples.
The bootstrap.php file ensures all the libraries are referenced correctly and also sets up some basic Java compatibility measures to aid in porting. It sort of serves as an application framework which the examples all depend on.
One other thing you'll notice is that I split up the two supporting library directories by category.
- The library/custom directory has custom written classes that supports the examples using custom written PHP.
- The library/java directory has classes that mimic Java as much as is needed for the examples to work.
How to Run the Command Line Examples
Simply run the appropriate run script from the terminal / command line and then follow the directions. Easy!
To elaborate, to simply run an example (or all examples) via command line, you will use the interactive run.bat and run.sh scripts, depending on your OS. Windows users would run the .bat file, and Linux and Mac OS X users would run the .sh file. You may need to modify the .sh file permissions, adding the executable flag, for it to run.
Please note that in PHP, you'll want to run scripts that start with the word "run". They will run the corresponding class. You'll also want to run them from the directory in which they reside, so that PHP can locate all the required files (some of the require statements use a relative path for inclusion, which is unfortunately sensitive to where the script is ran from).
With an IDE...
Or, if you'd like to have a better view of the code, you can run the examples from Eclipse. I like using the Eclipse PDT plugin, and just run the PHP scripts inside the IDE. You can then watch the debug output to see the example's print statements. I'll leave the setup of Eclipse PDT to the Eclipse site (www.eclipse.org/pdt). But if you follow this option, you'd want to make sure to install the Zend Debugger which lets you run PHP scripts directly on your Eclipse machine.
How to Run the MVC Example
To run the MVC example, you'll need a web server that is set up to run PHP. I won't cover that setup, since I assume you can handle that. But I will cover some pointers on how to get the code up and running with as little fuss as possible. The MVC app in PHP uses a very basic MVC framework but it comes with a lot of the same convention as other larger frameworks (like Zend Framework).
The basic idea is that you just need to serve the public folder in the example and have a way to http:// to that folder and have its index.php file served. Here's a more concrete set of steps based on experience:
Leave all existing code intact; the MVC application uses a lot of the existing source files and needs the entire HeadFirstDesignPatternsInPHP directory structure in order to work.
You'll want to serve the following path either in its own virtual host, or possibly as a symlink or virtual directory within an existing site:
For example, I like to define a virtual directory called "djview" in the root of the local site so that I can navigate to http://localhost/djview
Make sure your site serves index.php as a default document.
Make sure that the user running your Web server has write access to:
To preserve state across requests, the application will save data to disk in that location.
In Linux / MacOS, also keep in mind that Apache will want each directory that leads up to the public directory to have the executable permission set for the Web server user. This is especially important if you're using a virtual directory or symlink to pull the public directory into an existing site.
Special Notes by Pattern (lessons learned, special consideration, etc)
Adapter Pattern - Chapter 7
- This example doesn't map perfectly to PHP since Iterator and Enumeration interfaces and implementers either don't exist or don't map to Java, so some intermediate supporting code was needed to mimic Java.
- PHP has a build in Iterator interface, but it's not the same interface as Java so I renamed the interface to MyIterator for demo purposes.
- In the future, this example may be rewritten to use PHP's native Iterator.
Compound Patterns - Chapter 12
- Due to the use of midi, AWT, and Swing, this example isn't (yet) ported to PHP.
- This is a PHP adaptation of the DJView MVC example. The Java version uses servlets; in PHP, a simple MVC framework is used. The spirit of Java beans has been preserved and works well to keep the view and model separate. The model given to the view uses the virtual proxy pattern to wrap the model and only allow getter access to the underlying model. Refer to the How to Run the MVC Example section for information on running this.
- This example uses a FileModel, which basically writes the model state to disk as a serialized class. Normally, a database would be used, but I wanted to keep things simple to set up and get running quickly.
Command Pattern - Chapter 6
- Realized that unquoted, non-dollar sign prefixed text in PHP is treated as a string. I.e. echo stuff; echoes the string "stuff". Be careful when porting Java code to PHP!
Decorator Pattern - Chapter 3
- PHP5 doesn't let you convert a concrete method into an abstract one when using inheritance. Java allows this though.
Facade Pattern - Chapter 7
- PHP doesn't support method overloading, but it can determine the variable type of the argument and thus the same method can combine functionality. Either that or you'd have to provide two different method names in PHP. I chose to have only one method and change its behavior based on the argument type. After all, PHP is a dynamically-typed language.
Factory Pattern - Chapter 4
Abstract Factory Method
- The factory method pattern lets a class defer object instantiation to a subclass. Simple factory on the other hand just allows some class to do the instantiation.
Iterator Pattern - Chapter 9
- The code does not use the AlternatingDinerMenuIterator, ArrayIterator, or ArrayListIterator classes directly.
- Prior to PHP 5.3, namespaces aren't supported, so classes have to be named uniquely to avoid PHP "cannot redeclare class" errors.
Observer Pattern - Chapter 2
- PHP5 seems to have own Subject and Observer classes, renamed them to avoid any collisions. In the future, with PHP 5.3, namespaces can be used to keep them separate.
- Had to rewrite list of observers array usage; PHP arrays work different than Java's.
- Can also use "pull" method instead of "push" to control data flow more accurately.
Proxy Pattern - Chapter 11
- This example uses a custom written proxy, which does the work of Java's RMI.
- Creating proxy code in PHP using reflection is much simpler than in Java, thanks to PHP's dynamic nature.
- Due to the use of AWT and threads, this example isn't (yet) ported to PHP.
Singleton Pattern - Chapter 5
- Singleton isn't too difficult to implement. PHP5 can implement singletons very similarly to Java. Avoid the trap of trying to make a Singleton reusable class. Maybe create an interface for it instead.
- In PHP, make sure to disable the use of a class's __clone() method to prevent an existing instance from being cloned.
Double-Checked Locking & Threadsafe
- PHP5 doesn't natively support synchronized blocks, and so these examples are not easily ported to PHP. It doesn't matter anyway since PHP doesn't directly support threads, so synchronization isn't important for ensuring singletons work as expected within the same request.
Static (eagerly created instance)
- PHP5 cannot have expressions like new objects instantiated in member variables. Since the constructor is the only place to initialize such members, this Singleton pattern cannot be used in PHP5 (current version). Classic Singleton should be used instead.
- The visibility of methods that override their parent in PHP must be the same or weaker, unlike in Java where the access level can be overridden.
- This example doesn't work too well in PHP and gives an error due to the above.
State Pattern - Chapter 10
- PHP instance variables cannot be initialized with other class instance variables like they can in Java, so initialize them in the constructor.
Template Method Pattern - Chapter 8
- PHP doesn't support applets natively, so skipping the porting of this to PHP.
- PHP doesn't support Swing / AWT, though it does have GTK. Perhaps this can be ported in the future with a little effort!
- When porting this code, protected visibility is used, and then any failed access to a protected method or member results in the visibility being raised to public. Most of the time, protected works because classes are extended but sometimes, as with this example, some class in the same package will access a package private method.
- This was a fun one to port, because PHP provides a built-in usort function that works great with the Comparable interface. The custom function that gets passed into usort delegates its sorting to the compareTo() method of the two argument objects passed in, making for an elegant, native port.
All code is in the public domain for your use / education / enjoyment. Though, if you use any of the code in your projects or want to redistribute it, I'd just want to hear from you and/or receive a credit somewhere is about all. Thanks!
1.0.0 : 7/28/2009 - Initial release