13.8 Classes and Objects

PHP 5 provides comprehensive and robust support for object-oriented programming. If you've never heard of object-oriented programming, then you don't need to use any of these fancy features. But if you're coming to PHP from a language such as Java, you can structure your code in familiar ways. You can create interfaces; abstract classes; public, private, and protected properties and methods; constructors and destructors; overloaded property accessors and method dispatchers; and plenty of other OO goodies.

Chapter 2 of Upgrading to PHP 5 by Adam Trachtenberg (O'Reilly), lays out the many object-related changes in PHP 5. The PHP Manual covers classes and objects at http://www.php.net/manual/language.oop.php.

13.8.1 Object Basics

An object, in the programming world, is a structure that combines data about a thing (such as the ingredients in an entree) with actions on that thing (such as preparing the entree). Using objects in a program provides an organizational structure for grouping related variables and functions together.

Some words to know when working with objects are defined in the following list:

Class

A template or recipe that describes the variables and functions for a kind of object. For example, an Entree class would contain variables that hold its name and ingredients. The functions in an Entree class would be for things such as cooking the entree, serving it, and determining whether a particular ingredient is in it.

Method

A function defined in a class is called a method.

Property

A variable defined in a class is called a property.

Instance

An individual usage of a class. If you are serving three entrees for dinner in your program, you would create three instances of the Entree class. While each of these instances is based on the same class, they differ internally with different properties. The methods in each instance contain the same instructions, but probably produce different results because they each rely on the particular property values in each instance. Creating a new instance of a class is called "instantiating an object."

Constructor

A special method that is automatically run when an object is instantiated. Usually, constructors set up object properties and do other housekeeping that makes the object ready for use.

Static method

A special kind of method that can be called without instantiating a class. Static methods don't depend on the property values of a particular instance. PEAR DB uses a static method to create a database connection.

13.8.2 Creating a New Object

PEAR DB uses a static method to create a new object instance for you to use:

$db = DB::connect($dsn);

This calls the connect( ) method defined in the DB class. The connect( ) method is a static method: nothing in connect( ) depends on a specific instance of the DB class. The classname::method( ) syntax is how you call a static method. When you see two colons in a function name like that in a PHP program, think "static method call."

The other way to create a new object is with the new operator:

$dinner = new Entree( );

This makes the variable $dinner an instance of the class Entree. To pass arguments to a class's constructor, put them in the parentheses:

$dinner = new Entree('Chinese','spicy');

13.8.3 Accessing Properties and Methods

The -> ("arrow") operator, composed of a hyphen and a greater-than sign, is your road to the properties (variables) and methods (functions) inside an object. To access a property, put the arrow after the object's name and put the property after the arrow:

print $dinner->price;
$todays_fat = $todays_fat + $dinner->fat;
print 'To eat: '. strtoupper($dinner->name);

To call a method, put the method name after the arrow, followed by parentheses:

$dinner->prepare( );
$ingredients = $dinner->get_ingredients( );

You can pass arguments to a method just like a regular function:

$has_pineapple = $dinner->contains('Pineapple');
$dinner->add_ingredient('Ginger Root');
$dinner->serve('Alice','Bob','Charlie');

Note that the arrow operator used to access properties and methods is different than the operator-separating array keys and values in array( ) or foreach( ). The array arrow has an equals sign: =>. The object arrow has a hyphen: ->.