Declaring a Class

To design your program or code library in an object-oriented fashion, you'll need to define your own classes, using the class keyword.
A class definition includes the class name and the properties and methods of the class. Class names are case-insensitive and must conform to the rules for PHP identifiers. The class name stdClass is reserved. Here's the syntax for a class definition:

class classname [ extends baseclass ]
    {
        [ var $property [ = value ]; ... ]

        [ function functionname (args) {
              // code
          }
          ...
        ]
    }

Declaring Methods

A method is a function defined inside a class. Although PHP imposes no special restrictions, most methods act only on data within the object in which the method resides. Method names beginning with two underscores (_ _) may be used in the future by PHP (and are currently used for the object serialization methods _ _sleep( ) and _ _wakeup( ), described later in this chapter, among others), so it's recommended that you do not begin your method names with this sequence.

Within a method, the $this variable contains a reference to the object on which the method was called. For instance, if you call $rasmus->birthday( ) inside the birthday( ) method, $this holds the same value as $rasmus. Methods use the $this variable to access the properties of the current object and to call other methods on that object.

Here's a simple class definition of the Person class that shows the $this variable in action:

class Person { var $name; function get_name ( ) { return $this->name; } function set_name ($new_name) { $this->name = $new_name; } }

As you can see, the get_name( ) and set_name( ) methods use $this to access and set the $name property of the current object.

To declare a method as a static method, use the static keyword. Inside of static methods the variable $this is not defined. For example:

class HTML_Stuff {
static function start_table( ) {
echo "<table border='1'>\n";
 } static function end_table ( ) {
echo "</table>\n";
}
}
HTML_Stuff::start_table( ); // print HTML table rows and columns HTML_Stuff::end_table( );

Declaring Properties

In the previous definition of the Person class, we explicitly declared the $name property. Property declarations are optional and are simply a courtesy to whoever maintains your program. It's good PHP style to declare your properties, but you can add new properties at any time.

Here's a version of the Person class that has an undeclared $name property:

    class Person {
        function get_name (  )
        {
            return $this->name;    }

        function set_name ($new_name) {
            $this->name = $new_name;
        }
    }

You can assign default values to properties, but those default values must be simple constants:

    var $name = 'J Doe';       // works
    var $age  = 0;             // works
    var $day  = 60*60*24;      // doesn't work

Using access modifiers, you can change the visibility of properties. Properties that are accessible outside the object's scope should be declared public; properties on an instance that can only be accessed by methods within the same class should be declared private.