PHP means, enclosing the internal details of the object to protect from external sources. It describes, combining the class, data variables and member functions that work on data together within a single unit to form an object.

PHP program for encapsulation: The methods or functions in the following program are update password and check the course name. user class defines all the operations related to users.

<?php
  
// PHP program to implements encapsulation
class user {
  
    private $userId;
    private $pwd;
      
    // Update user password
    public function updatePwd($userId, $pwd) {
          
        // Write function body
        echo("Function to update password '"
                . $pwd . "' for user " . $userId);
          
        echo "<br>";
    }
  
    // Check account balance
    public function courseName($userId) {
          
        // Write function body
        echo("Function to check course name of user "
                . $userId);
        echo "<br>";
    }
}
  
$obj = new user();
$obj -> updatePwd('user1', 'abhi2');
$obj -> courseName('PHP');
  
?>

Output:

Function to update password ‘abhi2’ for user user1
Function to check course name of user PHP

Advantage of Encapsulation :

• It reduces human errors.
• It makes easier to understand.
• It’s protects an object from unwanted access by clients.
• Encapsulation reduces human errors.

Inheritance:

• Accessing the features of one class from another class.
• Inheritance allows you to write the code in the parent class and use it in both parent and child classes.
• The parent class is also called a base class or super class. And the child class is also known as a derived class or a subclass.

Declaration of Child Class :

class ChildClassName extends ParentClassName
{
members of Child class
};

Type of Inheritance in php:

• Single Inheritance
• Multi-level Inheritance
• Hierarchical Inheritance

Single Inheritance:

If a class is derived from one base class (Parent Class), it is called Single Inheritance.

Syntax:

class Father 
  { 
	 members of class Father 
  }; 

class Son extends Father 
 { 
	 members of class Son 
 }; 

Example of single level Inheritance:

<?php

    class Father
     {
         public $a;
         public $b;
         function getvalue($x, $y)
         {
             $this->a = $x;
             $this->b = $y;
         } 
     }
     class son extends Father{
       function display()
         {
             echo"value of A:$this->a <br>";
             echo"value of B:$this->b <br>";

        }
     }
     $obj = new son;
     $obj->getvalue(10,20);
     $obj->display();

    ?> 

Multi-level Inheritance:

In multi-level inheritance, the class inherits the feature of another derived class (Child Class).

syntax:

class Father 
  { 
 	members of class Father 
  }; 

class Son extends Father 
  { 
 	members of class Son 
  }; 

class GrandSon extends Son 
  { 
 	members of class GrandSon 
  }; 

Example of Multi-level Inheritance:

<?php
class Father
     {
         public $a;
         public $b;
         function getvalue($x, $y)
         {
             $this->a = $x;
             $this->b = $y;
         } 
     }
     class son extends Father
     {
         public $c=30;
         public $sum;
         function add()
         {
             $this->sum=$this->a+$this->b+$this->c;
             return $this->sum;
         }
     }
         class grandson extends son
         {
            function display()
             {
             echo "value of A :$this->a <br>";
             echo "value of B :$this->b <br>";
             echo "value of C :$this->c <br>";
             echo "value of sum : $this->sum <br>";
             }
        }
     $obj = new grandson;
     $obj->getValue(10,20);
     $obj->add();
     $obj->display();
?>

Hierarchical Inheritance:

class Father 
  { 
 	members of class Father 
  }; 

class Son extends Father 
  { 
 	members of class Son 
  }; 

class Daughter extends Father 
  { 
 	members of class Daughter 
  }; 

Example of Hierarchical Inheritance:

<?php
 class Father
    {
        public $a;
        public $b;
        function getvalue($x, $y)
        {
            $this->a = $x;
            $this->b = $y;
        } 
    }
    class son extends Father
    {
        public $c=30; 
        public $sum;
        function add()
        {
            $this->sum=$this->a+$this->b+$this->c;
            return $this->sum;
        }
        function display(){
            echo "value of A:$this->a <br>";
            echo "value of b:$this->b <br>";
            echo "value of c:$this->c <br>";
            echo "sum: $this->sum  <br>";
        }
    }   
    class Daugther extends Father

    {
        public $c=30;
        public $multi;
        function multiply()
        {
            $this->multi=$this->a*$this->b*$this->c;
            return $this->multi;
        }
        function display(){
            echo "value of A:$this->a <br>";
            echo "value of b:$this->b <br>";
            echo "value of c:$this->c <br>";
            echo "multipication: $this->multi";
        }
    }  
        $objs = new son;
        $objD = new Daugther;
        $objs->getValue(10,20);
        $objs->add();
        $objs->display();
        $objD->getValue(10,20);
        $objD->multiply();
        $objD->display();
    ?>    

polymorphism:

Polymorphism means “many forms”, and it occurs when we have many classes that are related to each other by inheritance

Real life example of polymorphism: A person at the same time can have different characteristics. Like a man at the same time is a father, a husband, an employee. So the same person possesses different behavior in different situations. This is called polymorphism.

Example of polymorphism:

<?php

class base

{

function add($a,$b)

{

$res=$a*$b;

echo "Multiplication = ".$res;

}

}


class child extends base

{

function add($a,$b)

{

$res=$a+$b;

echo "Sum  = ".$res;

}

}

 $obj= new child();

 $obj->add(1000,500);
 
?>

Output:
Sum = 1500