Classes and object orientation

Classes are used to define a real world thing, such as a person or a book. Once the class of thing has been defined then it can be instantiated.

An example class is below.

class Book
{
private $title;
private $author;
private $isbn;
private $reserved;

public function Reserve()
{
$this->reserved = true;
}
}

As you can see the the class is defined by using the class keyword followed by the name of the class which can be any non-reserved word in PHP. The variables and the function are all defined with the visibility modifier.

You will notice that the pseduo variable $this is used in the class. This variable allows functions and variables to be called from inside a class.

A class can be instantiated and called as shown below using the new

//Creates a instance of a class
$book = new Book();
//Calls the reserve function of the book object.
$book->Reserve();

Classes can also contain static functions. A static function doesn't require the class to be instantiated as a normal object method does. However a static method can't use any of the objects non-static variables.

class Book
{
private $title;
private $author;
private $isbn;
private $reserved;
private static $bookTypes = array('Reference', 'Fiction');

public function Reserve()
{
$this->reserved = true;
}

public static function GetBookType($index)
{
return self::$bookTypes[$index];
}
}

As you can see above the static function can use static class variables such as $bookTypes. A static variable or function can be accessed either via the self keyword or the class name inside the class and just the class name from outside it, as shown below.

//This prints Reference
echo Book::GetBookType(1);

A default value for a class variable must be a constant expression, so no concatanation or creating new classes!

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Written by Dominic Skinner
Last Updated: 2009-05-21 08:17:33