PHP | Common terminology in OOP

Object Oriented Programming is a very important concept for programmers to solve real-life problems. This article explaining the concept of object-oriented programming and its features.

Object: Everything in this world is surrounded with objects. Table, chair, monitor, cellphone everything is an object. There are two things in an object to remember to solve real-life problems. One is attribute and the other one is behavior. If we talk about monitor so…model number, screen size all these are attributes. On the other hand, the features like volume up or volume down are behavior for the monitor. In programming, variables are attributes and functions are behavior.

Example:

<?php  

// Class definition 

class TV { 

    // Member variables 

    public $model = 'xyz'; 

    public $volume = 1; 

    // Member Functions 

    function volumeUp() { 

        $this->volume++; 

    } 

    function volumeDown() { 

        $this->volume--; 

    } 
} 

// Create new object 

$tv_one = new TV; 

// Calling member function 

$tv_one->volumeUp(); 

echo $tv_one->volume; 

?>

Output:

In the above example, creating an object $tv_one of TV class and implementation of function showing the behavior of the object. Initially $tv_one->volume was 1. After calling its function volume has been increased and displaying the updated result. $this refers to the particular or current object. You can create multiple objects and show it’s behavior. Implementing this code will have the benefit of code re-usability and code will be easy to manage in the future.

Constructor function: Constructor function is an special function for which do not need to call the function like earlier we were doing after creating an object.

Example:

<?php  

// Class definition 

class TV { 

    // Member variables 

    public $model; 

    public $volume; 

    // Member Functions 

    function volumeUp() { 

        $this->volume++; 

    } 

    function volumeDown() { 

        $this->volume--; 

    } 

    function __construct($m, $v) { 

     $this->model = $m;  

     $this->volume= $v; 

   } 
} 

// Create new object 

$tv = new TV('xyz', 2); 

echo $tv->model; 

?>

Output:

xyz

In the above example, assigned the value of model and volume when we are creating the object. This is the benefit of using a constructor function. You don’t need to change the model number for each and every project you will be making. All need to do is to include this file and when you need to create an object of TV and assign its value you can do that instantly while creating its object.

Inheritance: In a simple word, inheritance means inheriting the properties or functionalities from one class to another class. Model and volume will find in all kinds of television but suppose you need to add additional feature timer. The timer can not be included in all the television. So it will use inheritance for that television which includes timer property. Use ‘extends’ keyword to implement inheritance.

Example:

<?php  

// Class definition 

class TV { 

    // Member variables 

    public $model; 

    public $volume; 

    // Member Functions 

    function volumeUp() { 

        $this->volume++; 

    } 

    function volumeDown() { 

        $this->volume--; 

    } 

    function __construct($m, $v) { 

        $this->model = $m;  

        $this->volume= $v; 

    } 
} 

class TvWithTimer extends TV { 

    public $timer = true; 
} 

// Create new object 

$tv = new TvWithTimer('xyz', 2); 

echo $tv->model; 

?>

Output:

xyz

Here, TVWithTimer class is a child class for TV class, whereas TV class is a parent class. The TVWithTimer class inherited all the variables and functions from its parent class so all the variables and functions which are in TV class will be available in TvWithTimer. So create multiple classes and inherit the property from TV class. Another important thing is Method Overriding. If defining the constructor function in child class after inheriting all the properties from parent class then the priorities will be given to child class constructor instead of a parent class.

class plazmaTv extends TV { 

     function __construct { 

      }   
} 

$plazma = new plazmaTv; 

echo $plazma->model;

Here, no output will display and the screen will be blank because the child class constructor is being called and it has overridden the parent class constructor. The same logic also applies for variables and you can override the variables in a child class as well.

Encapsulation: Encapsulation is all about wrapping the data and method in a single unit. Object-Oriented programming sets the access level for member functions or variables. It means from where it needs to be accessed. There are three kinds of access levels or access specifiers.

Public: Public functions or variables can only be accessed from anywhere. In all the above examples, we are able to access the public functions and variables outside the class and everyone can call it.

Private: Private functions or variables can only be accessed by the class who created it.

Example:

<?php  

// Class definition 

class TV { 

    // Member variables 

    private $model; 

    public $volume; 

    // Member Functions 

    function volumeUp() { 

        $this->volume++; 

    } 

    function volumeDown() { 

        $this->volume--; 

    } 

    function __construct($m, $v) { 

        $this->model = $m;  

        $this->volume= $v; 

    } 
} 

// Create new object 

$tv = new TV('xyz', 2); 

echo $tv->model; 

?>

Output:

Cannot access private property TV::$model in /home/65af96133e297a6da6c1ad3d75c9ff46.php:28

The error will display after executing the above code. It means the function or variable not accessible outside the class.

Protected: Protected functions or variables can only be used by the class who created and it’s child class means all the classes which inherit the property of parent class having protected method or variable.

Example:

<?php  

// Class definition 

class TV { 

    // Member variables 

    protected $model; 

    public $volume; 

    // Member Functions 

    function volumeUp() { 

        $this->volume++; 

    } 

    function volumeDown() { 

        $this->volume--; 

    } 

    function getModel() { 

        return $this->model; 

    } 

    function __construct($m, $v) { 

        $this->model = $m;  

        $this->volume= $v; 

    } 
} 

class plazma extends TV { 

    function getModel() { 

        return $this->model; 

    } 
} 

// Create new object 

$tv = new plazma('xyz', 1); 

echo $tv->getModel(); 

?>

Output:

xyz

Abstract Classes: Like the keyword is saying, Abstract means not completed. Abstract classes are those classes where functions are declared but not implemented.

Example:

<?php  

// Abstract class 

abstract class ABC { 

    public abstract function xyz(); 
} 

class def extends ABC { 

    public function xyz() { 

        echo "Welcome to GeeksforGeeks"; 

    } 
} 

// Create new object 

$obj = new def; 

echo $obj->xyz(); 

?>

Output:

Welcome to GeeksforGeeks

Here, we created an abstract class that has one abstract method but we haven’t implemented the method in ABC base class. All the classes which will extend ABC class will have the implementation for function XYZ. Here you can not make the object for ABC class. ABC class works as a base class for all it’s child classes and the implementation or object creation will be done for child classes. Declaring a class abstract class is enforcement that you don’t want to create a direct object for that class.

Example:

<?php  

abstract class BaseEmployee { 

    protected $firstname; 

    protected $lastname; 

    public function getFullName() { 

        return $this->firstname. " " .$this->lastname; 

    } 

    public abstract function getMonthlySalary(); 

    public function __construct($f, $l) { 

        $this->firstname = $f; 

        $this->lastname = $l; 

    } 
} 

class FullTimeEmployee extends BaseEmployee { 

    protected $annualSalary; 

    public function getMonthlySalary() { 

    return $this->annualSalary / 12; 

    } 
} 

class ContractEmployee extends BaseEmployee { 

    protected $hourlyRate; 

    protected $totalHours; 

    public function getMonthlySalary() { 

        return $this->hourlyRate * $this->totalHours; 

    } 
} 

$fulltime = new FullTimeEmployee('fulltime', 'Employee'); 

$contract = new ContractEmployee('Contract', 'Employee'); 

echo $fulltime->getFullName(); 

echo $contract->getFullName(); 

echo $fulltime->getMonthlySalary(); 

echo $contract->getMonthlySalary(); 

?>

Output:

fulltime EmployeeContract Employee00

Interfaces: There is a limitation for inheritance in PHP that is you can not extend multiple classes. Through Interface you can achieve multiple inheritance property. Interfaces also worked like abstract classes. The ‘implements’ keyword is used for interfaces. There is a difference between abstract class and interface class. In abstract class define variables but in the interface, the class can not define variables. It can also not create constructor function in interfaces. It can not declare a private or protected function.

Example:

<?php  

interface a { 

    public function abc(); 
} 

interface b { 

    public function xyz(); 
} 

class c implements a, b { 

    public function abc() { 

        echo 'GeeksforGeeks '; 

    } 

    public function xyz() { 

        echo 'A computer science portal'; 

    } 

} 

// Create an object 

$obj = new c; 

$obj->abc(); 

$obj->xyz(); 

?>

Output:

GeeksforGeeks A computer science portal

Static Members: Static variables or static functions are directly related with class. It can access static variables or functions directly with the class names. Use the ‘static’ keyword to declare a static function or static variable.

Example:

<?php  

// Class definition 

class abc { 

    public static $data = 'GeeksforGeeks '; 

    public static function xyz() { 

        echo "A computer science portal"; 

    } 
} 

echo abc::$data; 
abc::xyz(); 

?>

Output:

GeeksforGeeks A computer science portal

Example: This example count how many objects we have created for a class.

<?php  

class abc { 

    public static $objectCount = 0; 

    public static function getCount() { 

        return self::$objectCount; 

    } 

    public function __construct() { 

        self::$objectCount++; 

    } 
} 

$a = new abc(); 

$b = new abc(); 

$c = new abc(); 

$d = new abc(); 

echo abc::getCount(); 

?>

Output:

Note: If created object instead of static using static function or variable every time the output will be 1. So static variable or keyword is related to class not object.

Late Static Binding: In PHP 5.3 a concept of late static binding was introduced.

Example:

<?php  

class DB { 

    protected static $table = ' basetable'; 

    public function select() { 

        echo 'SELECT * FROM' .self::$table; 

    } 

    public function insert() { 

        echo 'INSERT INTO' .self::$table; 

    } 
} 

class abc extends DB { 

    protected static $table = 'abc'; 
} 

$abc = new abc; 

$abc->select(); 

?>

Output:

SELECT * FROM basetable

Here, class DB extends class abc, but when running this code during compilation time it’s taking the table name from abc class, not DB. During the compilation time, the PHP interpreter immediately assigns the value from the parent class. In order to get the variable from the child, the class interpreter needs to assign the value during runtime instead of compilation time which is happening in the above example.

echo 'SELECT * FROM' .self::$table;

echo 'INSERT INTO' .self::$table;

Here when the interpreter will get the request it won’t assign the value during compilation time. It will be in pending for runtime. It will assign the value during runtime time and the output will be SELECT * FROM abc

Polymorphism: Polymorphism means many types or many forms. It means suppose you have created an interface also classes that will implement this interface class. These classes will have different functionality and they all will share this common interface.

Create an interface with LoggerInterface name in LoggerInterface.php file.

<?php  

interface LoggerInterface { 

   public function log($message); 
} 
?> 

Now creating a file EmailLogger.php which will implement this interface.

<?php  

class EmailLogger implements LoggerInterface { 

  public function log($message) { 

      echo "Logging message to email: $message"; 

  } 
} 
?> 

Create another file FileLogger.php and implement the interface class.

<?php  

class FileLogger implements LoggerInterface { 

    public function log($message) { 

        echo "Logging message to file: $message"; 

    } 
} 
?>

Another file is DBLogger.php

<?php  

class DBLogger implements LoggerInterface { 

    public function log($message) { 

        echo "logging message to DB: $message"; 

    } 
} 
?> 

Now, create a UserProfile.php file where call all these files by only defining interface name using type hinting. Do not need to define the class name for calling each and every class created in a file.

<?php  

class UserProfile { 

    private $logger; 

    public function createUser() { 

        echo "creating user"; 

        $this->logger->log("User Created"); 

    } 

    public function updateUser() { 

        echo "updating user"; 

        $this->logger->log("User Updated"); 

    } 

    public function deleteUser() { 

        echo "deleting user"; 

        $this->logger->log("Deleting User"); 

    } 

    public function __construct(LoggerInterface $logger)  

    { 

        $this->logger = $logger; 

    } 

} 

?>

Now, create a file index.php

<?php  

function __autoload($class) { 

    // classes is a directory where all 

    // the above files are placed. 

    include_once "classes/$class.php"; 
} 

function getLogger($type) { 

    switch($type) { 

        case 'email': 

            return new EmailLogger(); 

            break; 

        case 'database': 

            return new DBLogger(); 

            break; 

        case 'file': 

            return new FileLogger(); 

            break; 

    } 
} 

$logger = getLogger('database'); 

$profile = new UserProfile($logger); 

$profile->createUser(); 

?>

Output:

Creating User. Logging message to DB: User Created

So, from this example create multiple files and define their functionality. All need to implement the interface class and just give the interface name in type hinting name as I have defined in UserProfile.php file.

Traits: Traits was introduced in PHP5.4. It is the mechanism for code reuse in single inheritance language. The ‘trait’ keyword is used to define it.

Example:

<?php  

class abc { 

    public function test() { 

        echo "test from class ABC"; 

    } 
} 

trait test {  

    public function test2() { 

        echo "test2 method of test trait"; 

    } 
} 

class one extends abc { 

    use test; 
} 

class two extends abc { 

    use test; 
} 

class three extends abc { 

} 

class four extends abc { 

} 

class five extends abc { 

} 

$obj = new one; 

$obj->test2(); 

?>

Output:

test2 method of test trait

Here, taking a class abc and its functionality in all the child classes. So accessing the properties of abc class in all the child classes. Now consider a scenario where we need another function to be accessed only in class one and two but not in the rest of the classes. If you will define that function in the same class abc it will be available in all the classes which are extending class abc. So the solution is we can define that method in trait.
Trait is also similar to the classes. Here, define test2 function in trait test and use this trait by using ‘use’ keyword in child class. So we have used this trait only in class one and two because we need to access this method in class one and two.

Method Overriding in Traits: If defining the same method implemented in class and in trait also then preference will be given to the trait class method implementation. It means a trait class method will override the parent class method.

Example:

<?php  

class Base { 

    public function abc() { 

        echo "ABC method from base class"; 

    } 
} 

Trait Test { 

    public function abc() { 

        echo "ABC method from test trait"; 

    } 
} 

class child extends Base { 

    use Test; 
} 

$obj = new Child; 

$obj->abc(); 

?>

Output:

ABC method from test trait

So, the order of preference is child class method > trait class method > parent class method.

Access level in traits: You can change the access level of traits in child class.

Example:

<?php 

trait abc { 

    public function xyz() { 

        echo "xyz method from trait abc"; 

   } 
} 

class test { 

    use abc { 

        abc::xyz as public abcXyz; 

    } 
} 

$obj = new test; 

$obj->abcXyz(); 

?>

Output:

xyz method from trait abc

Namespaces: In PHP, class can not be redeclare. It means a class can not be declared more than once. It will display an error message. To solve this problem the concept of a namespace is used.

Example:

<?php  

namespace def { 

    class xyz { 

        public function __construct() { 

            echo "XYZ from DEF namespace"; 

        }  

    } 
} 

namespace { 

    class xyz { 

        public function __construct() { 

            echo "XYZ from Global namespace"; 

        } 

    } 

    $obj  = new def/xyz(); 
} 

?>

Output:

XYZ from DEF namespace

By default, all the classes are part of the global namespace. The global namespace declared with only keyword ‘namespace’ (no need to define the name of the namespace). The code can not be declared outside the namespace. Also, namespace should be the first line in your code if you are declaring a class in it.
There is alternate way as well to call the function from def class. just replace the code like below when you create an object.

use def\xyz as def;
$obj = new def();

The output will be the same. You can also make sub-namespace for a namespace. All you just need to do when you declare it is below the line.

namespace def\ghi

Type Hinting: In PHP, don’t need to define the data type of variable you are declaring. It automatically defines the data type when creating a variable, but sometimes when you receive a variable, there you need to define what kind of variable you are receiving.

Example:

<?php  

// Function definition 

function test(array $arr) { 

    foreach($arr as $k => $v) { 

        echo $k . "  " . $v . "\n"; 

    } 
} 

// Declare an array 

$array = ['abc'=>'ABC', 'xyz'=>'XYZ']; 

// Function call 

test($array); 

?>

Output:

abc  ABC

xyz  XYZ

So, when receiving the variable and defined that it should be an array type. If you will parse string type data $array ='abcdefgh' then you will get a fatal error and the code won't execute. You can also do type hinting for class name and interface.

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