oop in php lecture 2

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Object Oriented Programming in

PHP


Understanding Object-Oriented Programming

• Object-oriented programming is a style of coding that allows developers to group similar tasks into classes.

• “don’t repeat yourself” (DRY) approach, and easy-to-maintain.• Usually only one change is required to update the code.• Maintaining code where data is declared over and over again.

http://en.wikipedia.org/wiki/Don


Understanding Objects and Classes

• Before you can get too deep into the finer points of OOP, a basic understanding of the differences between objects and classes is necessary.


Recognizing the Differences Between Objects and Classes

CLASS

•A class, for example, is like a blueprint for a house.•It defines the shape of the house on paper, with relationships between the different parts of the house clearly defined and planned out.•Even though the house doesn’t exist.

OBJECT

•An object, then, is like the actual house built according to that blueprint.•The data stored in the object is like the wood, wires, and concrete that compose the house.•when it all comes together, it becomes an organized, useful house.•More than one object can be built from the same class.

Structuring Classes

• The syntax to create a class is pretty straightforward• Declare a class using the class keyword, followed by the name

of the class and a set of curly braces ({}):• Syntax: <?php

class MyClass{

// Class properties and methods go here}

?>

• After creating the class, a new class can be instantiated and stored in a variable using the new keyword

$obj = new MyClass;• To see the contents of the class, use var_dump():• var_dump($obj); • object(MyClass)#1 (0) { }

Defining Class Properties

• To add data to a class, properties, or class-specific variables, are used.

• These work exactly like regular variables, except they’re bound to the object and therefore can only be accessed using the object.

• The keyword public determines the visibility of the property, which you’ll learn about a little later.

• To read this property and output it to the browser, reference the object from which to read and the property to be read.

• echo $obj->prop1;

<?php class MyClass { public $prop1 = "I'm a class property!"; } $obj = new MyClass; var_dump($obj); ?>

Defining Class Properties Cont…

• Because multiple instances of a class can exist, if the individual object is not referenced, the script would be unable to determine which object to read

• The use of the arrow (->) is an OOP construct that accesses the contained properties and methods of a given object.

<?php class MyClass { public $prop1 = "I'm a class property!"; } $obj = new MyClass; echo $obj->prop1; // Output the property ?>

Defining Class Methods• Methods are class-specific functions.• Individual actions that an object will be able to perform are

defined within the class as methods.• For instance, to create methods that would set and get the

value of the class property $prop1, add the following to your code. <?php

class MyClass { public $prop1 = "I'm a class property!"; public function setProperty($newval) { $this->prop1 = $newval; } public function getProperty() { return $this->prop1 . " "; } } $obj = new MyClass; echo $obj->prop1; ?>

Defining Class Methods Cont…• OOP allows objects to reference themselves using $this. When

working within a method.• use $this in the same way you would use the object name

outside the class.• To use these methods, call them just like regular functions,

but first, reference the object they belong to. <?php class MyClass { public $prop1 = "I'm a class property!"; public function setProperty($newval) { $this->prop1 = $newval; } public function getProperty() { return $this->prop1 . " "; } } $obj = new MyClass; echo $obj->getProperty(); // Get the property value $obj->setProperty("I'm a new property value!"); // Set a new one echo $obj->getProperty(); // Read it out again to show the change ?>

Defining Class Methods Cont…• The power of OOP becomes apparent when using multiple

instances of the same class.<?php class MyClass { public $prop1 = "I'm a class property!"; public function setProperty($newval) { $this->prop1 = $newval; } public function getProperty() { return $this->prop1 . " "; } } $obj = new MyClass; $obj2 = new MyClass; // Get the value of $prop1 from both objects echo $obj->getProperty(); echo $obj2->getProperty(); // Set new values for both objects $obj->setProperty("I'm a new property value!"); $obj2->setProperty("I belong to the second instance!"); // Output both objects' $prop1 value echo $obj->getProperty(); echo $obj2->getProperty(); ?>

Magic Methods in OOP• PHP also provides a number of magic methods, or special

methods that are called when certain common actions occur within objects.

Using Constructors and Destructors• To handle this, PHP provides the magic method __construct(),

which is called automatically whenever a new object iscreated.

• __CLASS__ returns the name of the class in which it is called; this is what is known as a magic constant.

<?php class MyClass { public function __construct() { echo 'The class "', __CLASS__, '" was initiated! '; } } // Create a new object $obj = new MyClass; ?>

http://us3.php.net/manual/en/language.constants.predefined.php

Magic Methods in OOP Cont…Using Constructors and Destructors

•To call a function when the object is destroyed, the __destruct() magic method is available.•This is useful for class cleanup (closing a database connection, for instance).

<?php class MyClass { public $prop1 = "I'm a class property!"; public function __construct() { echo 'The class "', __CLASS__, '" was initiated! '; } public function __destruct() { echo 'The class "', __CLASS__, '" was destroyed. '; } } // Create a new object $obj = new MyClass;

// Output a message at the end of the file echo "End of file. "; ?>

Magic Methods in OOP Cont…Using Constructors and Destructors

•When the end of a file is reached, PHP automatically releases all resources.•To explicitly trigger the destructor, you can destroy the object using the function unset():

<?php class MyClass { public $prop1 = "I'm a class property!"; public function __construct() { echo 'The class "', __CLASS__, '" was initiated! '; } public function __destruct() { echo 'The class "', __CLASS__, '" was destroyed. '; } } // Create a new object $obj = new MyClass; unset($obj); // Destroy the object // Output a message at the end of the file echo "End of file. "; ?>

Using Class Inheritance• Classes can inherit the methods and properties of another

class using the extends keyword.<?php class MyClass { public $prop1 = "I'm a class property!"; public function __construct() { echo 'The class "', __CLASS__, '" was initiated! '; } public function __destruct() { echo 'The class "', __CLASS__, '" was destroyed. '; } public function setProperty($newval) { $this->prop1 = $newval; } public function getProperty() { return $this->prop1 . " "; } }

class MyOtherClass extends MyClass { public function newMethod() { echo "From a new method in " . __CLASS__ . ". "; } } // Create a new object $newobj = new MyOtherClass; // Output the object as a string echo $newobj->newMethod(); // Use a method from the parent class echo $newobj->getProperty(); ?>

Overwriting Inherited Properties and Methods• To change the behavior of an existing property or method in

the new class, you can simply overwrite it by declaring it again in the new class:

<?php class MyClass { public $prop1 = "I'm a class property!"; public function __construct() { echo 'The class "', __CLASS__, '" was initiated! '; } public function __destruct() { echo 'The class "', __CLASS__, '" was destroyed. '; } public function setProperty($newval) { $this->prop1 = $newval; } public function getProperty() { return $this->prop1 . " "; } }

class MyOtherClass extends MyClass { public function __construct() { echo "A new constructor in " . __CLASS__ . ". "; } public function newMethod() { echo "From a new method in " . __CLASS__ . ". "; } } // Create a new object $newobj = new MyOtherClass; // Output the object as a string echo $newobj->newMethod(); // Use a method from the parent class echo $newobj->getProperty(); ?>

Preserving Original Method Functionality While Overwriting Methods

• To add new functionality to an inherited method while keeping the original method intact, use the parent keyword with the scope resolution operator (::)

• This outputs the result of both the parent constructor and the new class’s constructor.

class MyOtherClass extends MyClass { public function __construct() { parent::__construct(); // Call the parent class's constructor

echo "A new constructor in " . __CLASS__ . ". "; } public function newMethod() { echo "From a new method in " . __CLASS__ . ". "; } } // Create a new object $newobj = new MyOtherClass; ?>

Preventing from Overriding

• If you declare any method as a final method, it can't be overridden in any of its subclass.

• if you don't want someone to override your class methods, declare it as final.

• If you execute the above code, it will generate a fatal error because class SubClass tried to override a method in SuperClass which was declared as final.

<?class SuperClass{ public final function someMethod() { //..something here }}class SubClass extends SuperClass { public function someMethod() { //..something here again, but it wont run }}?>

Preventing from Extending

• Similar to a final method, you can declare a class as final.• It will prevent anyone from extending it.• So if you declare any class, as shown in following example, it

is no more extensible.

• If you execute the code above, it will trigger the Fatal error.

<?final class aclass{}class bclass extends aclass {}?>


Assigning the Visibility of Properties and Methods

• For added control over objects, methods and properties are assigned visibility.

• This controls how and from where properties and methods can be accessed.

• There are three visibility keywords.• Public• Protected• Private

• In addition to its visibility, a method or property can be declared as static, which allows them to be accessed without an instantiation of the class.

• Visibility is a new feature as of PHP 5.


Public Properties and Methods

• All the methods and properties you’ve used so far have been public.

• This means that they can be accessed anywhere, both within the class and externally.


Protected Properties and Methods• When a property or method is declared protected, it can only

be accessed within the class itself or in descendant classes (classes that extend the class containing the protected method).

• This means that they can be accessed anywhere, both within the class and externally.

• Declare the getProperty() method as protected in MyClass and try to access it directly from outside the class.

Protected Properties and Methods Cont…<?php class MyClass { public $prop1 = "I'm a class property!"; public function __construct() { echo 'The class "', __CLASS__, '" was initiated! '; } public function __destruct() { echo 'The class "', __CLASS__, '" was destroyed. '; } public function setProperty($newval) { $this->prop1 = $newval; } protected function getProperty() { return $this->prop1 . " "; } }

class MyOtherClass extends MyClass { public function __construct() { parent::__construct(); echo "A new constructor in " . __CLASS__ . ". ";

} public function newMethod() { echo "From a new method in " . __CLASS__ . ". "; } }

// Create a new object $newobj = new MyOtherClass; // Attempt to call a protected method echo $newobj->getProperty(); ?>

• Upon attempting to run this script, error will occur since protected methods or properties can not be accessed outside the class.


Private Properties and Methods• A property or method declared private is accessible only from

within the class that defines it.• This means that even if a new class extends the class that

defines a private property, that property or method will not be available at all within the child class.

• To demonstrate this, declare getProperty() as private in MyClass, and attempt to call callProtected() fromMyOtherClass.

Private Properties and Methods Cont…<?php class MyClass { public $prop1 = "I'm a class property!"; public function __construct() { echo 'The class "', __CLASS__, '" was initiated! '; } public function __destruct() { echo 'The class "', __CLASS__, '" was destroyed. '; } public function setProperty($newval) { $this->prop1 = $newval; } private function getProperty() { return $this->prop1 . " "; } }


} public function callProtected() { return $this->getProperty(); } } // Create a new object $newobj = new MyOtherClass; // Use a method from the parent class echo $newobj->callProtected(); ?>

• Upon attempting to run this script, error will occur since private methods or properties can not be accessed outside the class and also in child class.


Static Properties and Methods• A method or property declared static can be accessed without

first instantiating the class.• A static method and property can also be said as class

properties and methods.• You simply supply the class name, scope resolution operator,

and the property or method name.• To demonstrate this, add a static property called $count and a

static method called plusOne() to MyClass. Then set up a do...while loop to output the incremented value of $count as long as the value is less than 10.

Static Properties and Methods Cont…<?php class MyClass { public static $count = 0; public function __construct() { echo 'The class "', __CLASS__, '" was initiated! '; } public static function plusOne() { return "The count is " . ++self::$count . ". "; }

} do { // Call plusOne without instantiating MyClass echo MyClass::plusOne(); } while ( MyClass::$count < 10 ); ?>


} public function callProtected() { return $this->getProperty(); } }

• Note — When accessing static properties, the dollar sign($) comes after the scope resolution operator.


Static Properties and Methods Cont…

• When you load this script in your browser, the following is output.

The count is 1. The count is 2. The count is 3. The count is 4. The count is 5. The count is 6. The count is 7. The count is 8. The count is 9. The count is 10.

oop in php lecture 2

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