Methods

Put simply, methods are functions declared within a class. They are usually -- but not always -- called via an object instance using the object operator. Listing 5 adds a method to the Dictionary class and invokes it.

class Dictionary {
    public $translations = array();
    public $type ="En";

    function summarize() {
        $ret  = "Dictionary type: {$this->type}n";
        $ret .= "Terms: ".count( $this->translations )."n";
        return $ret;
    }
}

$en = new Dictionary();
$en->translations['TREE'] = "tree";
print $en->summarize();

It provides output of:

Dictionary type: En
Terms: 1

As you can see, the summarize() method is declared just as any function would be declared, except that is done within a class. The summarize() method is invoked via a Dictionary instance using the object operator. The summarize() function accesses properties to provide a short overview of the state of the object.

Notice the use of a feature new to this article. The $this pseudo-variable provides a mechanism for objects to refer to their own properties and methods. Outside of an object, there is a handle you can use to access its elements ($en, in this case). Inside an object, there is no such handle, so you must fall back on $this. If you find $this confusing, try replacing it in your mind with the current instance when you encounter it in code.

Classes are often represented in diagrams using the Universal Modeling Language (UML). The details of the UML are beyond the scope of this article, but such diagrams are nonetheless an excellent way of visualizing class relationships. Figure 1 shows the Dictionary class as it stands. The class name lives in the top layer, properties in the middle, and methods at the bottom.

The constructor

The PHP engine recognizes a number of "magic" methods. If they are defined, it invokes these methods automatically when the correct circumstances arise. The most commonly implemented of these methods is the constructor method. The PHP engine calls a constructor when the object is instantiated. It is the place to put any essential setup code for your object. In PHP V4, you create a constructor by declaring a method with the same name as that of the class. In V5, you should declare a method called __construct(). Listing 6 shows a constructor that requires a DictionaryIO object.

class Dictionary {
    public $translations = array();
    public $type;
    public $dictio;

    function __construct( $type, DictionaryIO $dictio ) {
        $this->type = $type;
        $this->dictio=$dictio;
    }

    //...

To instantiate a Dictionary object, you need to pass a type string and a DictionaryIO object to its constructor. The constructor uses these parameters to set its own properties. Here is how you might now instantiate a Dictionary object:

$en = new Dictionary( "En", new DictionaryIO() );

The Dictionary class is now much safer than before. You know that any Dictionary object will have been initialized with the required arguments.

Of course, there's no way yet to stop someone coming along later and changing the $type property or setting $dictio to null. Luckily, PHP V5 can help you there, too.