PHP Classes and Objects Introduction

PHP Classes and Objects

Introduction

Early versions of PHP were strictly procedural: you could define functions, but not objects. PHP 3 introduced an extremely rudimentary form of objects, written as a latenight hack. Back in 1997, nobody expected the explosion in the number of PHP pro‐grammers, or that people would write large-scale programs in PHP. Therefore, these limitations weren’t considered a problem.

Over the years, PHP gained additional object-oriented (OO) features; however, the development team never redesigned the core OO code to gracefully handle objects and classes. As a result, although PHP 4 improved overall performance, writing complex OO programs with it was still difficult, if not nearly impossible.

PHP 5 fixed these problems by using Zend Engine 2 (ZE2). ZE2 enables PHP to include more advanced object-oriented features, while still providing a high degree of backward compatibility to the millions of PHP scripts already written. Later versions of PHP 5 further enhanced PHP’s OO toolkit. Today, it’s capable of allowing developers to write fully featured OO applications.

If you don’t have experience with object-oriented programming, then you’re in for a bit of a surprise. Although some features allow you to do things more easily, many features actually restrict what you can do.

Even though it seems counterintuitive, these limitations actually help you quickly write safe code because they promote code reuse and data encapsulation. These key OO programming techniques are explained throughout the chapter. But first, here’s an introduction to object-oriented programming, its vocabulary, and its concepts.

A class is a package containing two things: data and methods to access and modify that data. The data portion consists of variables; they’re known as properties. The other part of a class is a set of functions that can use its properties—they’re called methods.

When you define a class, you don’t define an object that can be accessed and manipulated. Instead, you define a template for an object. From this blueprint, you create malleable objects through a process known as instantiation. A program can have multiple objects of the same class, just as a person can have more than one book or many pieces of fruit.

Classes also live in a defined hierarchy. Each class down the line is more specialized than the one above it. These specialized classes are called child classes, and the class they’re modifying is called the parent class. For example, a parent class could be a building.

Buildings can be further divided into residential and commercial. Residential buildings can be further subdivided into houses and apartment buildings, and so forth. The top most parent class is also called the base class.

Both houses and apartment buildings have the same set of properties as all residential buildings, just as residential and commercial buildings share some things in common.

When classes are used to express these parent-child relationships, the child class inherits the properties and methods defined in the parent class. This allows you to reuse the code from the parent class and requires you to write code only to adapt the new child to its specialized circumstances. This is called inheritance and is one of the major advantages of classes over functions. The process of defining a child class from a parent is known as subclassing or extending.

Classes in PHP are easy to define and create:

          class guest_book {

            public $comments;

            public $last_visitor;

            function update($comment, $visitor) {

            ...

            }

          }

The class keyword defines a class, just as function defines a function. Properties are declared using the public keyword. Method declaration is identical to function definition.

The new keyword instantiates an object:

          $gb = new guest_book;

Object instantiation is covered in more detail.

Inside a class, you can optionally declare properties using public. There’s no requirement to do so, but it is a useful way to reveal all the variables of the class. Because PHP doesn’t force you to predeclare all your variables, it’s possible to create one inside a class without PHP throwing an error or otherwise letting you know. This can cause the list of variables at the top of a class definition to be misleading, because it’s not the same as the list of variables actually in the class.

Besides declaring a property, you can also assign it a value:

          public $last_visitor = 'Donnan';

The right-hand side of this construct can only be a constant value:

          public $last_visitor = 'Donnan'; // okay

          public $last_visitor = 9; // okay

          public $last_visitor = array('Jesse'); // okay

          public $last_visitor = pick_visitor(); // bad

          public $last_visitor = 'Chris' . '9'; // bad

If you try to assign something else, PHP dies with a parse error.

To assign a nonconstant value to a variable, do it from a method inside the class:

          class guest_book {

               public $last_visitor;

               public function update($comment, $visitor) {

                   if (!empty($comment)) {

                      array_unshift($this->comments, $comment);

                      $this->last_visitor = $visitor;

                   }

               }

          }

If the visitor left a comment, you add it to the beginning of the array of comments and set that person as the latest visitor to the guest book. The variable $this is a special variable that refers to the current object. So to access the $last_visitor property of an object from inside that object, refer to $this->last_visitor.

To assign nonconstant values to variables upon instantiation, assign them in the class constructor. The class constructor is a method automatically called when a new object is created, and it is named __construct(), as shown:

          class guest_book {

               public $comments;

               public $last_visitor;

               public function __construct($user) {

                    $dbh = mysqli_connect('localhost', 'username', 'password', 'sites');

                    $user = mysqli_real_escape_string($dbh, $user);

                    $sql = "SELECT comments, last_visitor FROM guest_books WHERE user='$user'";

                    $r = mysqli_query($dbh, $sql);

                    if ($obj = mysqli_fetch_object($dbh, $r)) {

                         $this->comments = $obj->comments;

                         $this->last_visitor = $obj->last_visitor;

                    }

               }

          }

          $gb = new guest_book('stewart');

Constructors are covered.

Be careful not to mistakenly type $this->$size. This is legal, but it’s not the same as $this->size. Instead, it accesses the property of the object whose name is the value stored in the $size variable. More often than not, $size is undefined, so $this-> $size appears empty. For more on variable property names.

As of PHP, you can call a method or access a property directly upon object instantiation:

          $last_visitor = (new guest_book('stewart'))->last_visitor;

          $last_visitor = (new guest_book('stewart'))->getLastVisitor();

Besides using -> to access a method or member variable, you can also use ::. This syntax accesses static methods in a class. These methods are identical for every instance of a class, because they can’t rely on instance-specific data. There’s no $this in a static method. For example:

          class convert {

             // convert from Celsius to Fahrenheit

             public static function c2f($degrees) {

                 return (1.8 * $degrees) + 32;

             }

          }

          $f = convert::c2f(100); // 212

To implement inheritance by extending an existing class, use the extends keyword:

          class xhtml extends xml {

               // ...

          }

Child classes inherit parent methods and can optionally choose to implement their own specific versions. For example:

          class DB {

             public $result;

             function getResult() {

                 return $this->result;

             }

             function query($sql) {

                 error_log("query() must be overridden by a database-specific child");

                 return false;

             }

          }

          class MySQL extends DB {

             function query($sql) {

               $this->result = mysql_query($sql);

             }

          }

The MySQL class inherits the getResult() method unchanged from the parent DB class, but has its own MySQL-specific query() method. Preface the method name with parent:: to explicitly call a parent method:

           function escape($sql) {

               $safe_sql = mysql_real_escape_string($sql); // escape special characters

               $safe_sql = parent::escape($safe_sql); // parent method adds '' around $sql

               return $safe_sql;

           }