Abstract class in PHP

Concept
Abstract class cannot be instantiated, that is we can not create a object. Any class which has at-least one abstract function should be  declared as abstract. These abstract functions are only can declare, can not define here. But these are must be defined in classes which are inherited this abstract class. We can declare this abstract function as any type(public, protected or private) in abstract class, but it should be same or less restricted type in inherited class(For example declared as protected may had public in derived class). And number of arguments also should be same(May had additional optional argument in derived class.).

Clear Example
<?php
abstract class A                          // Abstract class.
{
abstract protected function fun1();   // Declaring abstract function as protected.
abstract public function fun2($i);  // Declaring abstract function as public with one argument.
abstract public function fun3($i,$j); // Declaring abstract function as public with two arguments.

public function fun4()  // Declaring and defining a normal function in abstract class.
{
echo 'fun4 definition from class A(abstract class).<br />';
}
}

class B extends A
{
public function fun1()  // Definiion for a abstract function.
{
echo 'fun1 definition from class B.<br />';
}

public function fun2($i)  // Definiion for a abstract function with exact no.of arugument.
{
echo 'fun2 definition from class B.<br />';
}

public function fun3($i,$j,$k=30)  // Definiion for a abstract function with optional arugument.
{
echo 'fun3 definition from class B.<br />';
}
}

$b=new B();
echo $b->fun1();
echo $b->fun2(10);
echo $b->fun3(10,20);
echo $b->fun4();
?>

Abstract vs Interface
1. Abstract is a class but interface not. So memory allocation for abstract is high.
2. Abstract class can had normal functions and variables. But interface cannot.
3. Functions on interface must be public but in abstract class may had anything.
4. A class can inherit only one abstract class but can inherit more than one interface.

Interface in PHP

Basic Concept
Interface is declared as same as class, except interface having only function declarations without any definition. A class which implementing a interface must be defined all functions which are declared in interface. We can implement a interface using the keyword implements.

Example 1: Simple Interface Concept.
<?php
interface BasicAction
{
        public function grow();
}

class Man implements BasicAction
{
        public function speak()
        {
               echo 'Will Speak';        
        }

        public function grow()
        {
               echo 'Will Grow';        
        }
}

$obj=new Man();
$obj->speak();
$obj->grow();
?>

Example 2: Extend a interface on another interface.

interface BasicAction
{
        public function grow();
}

interface Animal extends BasicAction
{
        public function walk();
}

class Deer implements Animal
{
        public function walk()
        {
               echo 'Will Walk';        
        }

        public function grow()
        {
               echo 'Will Grow';        
        }
}

Example 3: Multiple inheritance of interfaces on a class

interface A
{
        public function fun1();
}

interface B
{
        public function fun2();
}

class Test implements A, B
{
        public function fun1()
        {
               echo 'Fun 1 Definition';
        }

        public function fun2()
        {
               echo 'Fun 1 Definition';
        }
}

Interface Characters
1. All functions on interface must be public.
2. All functions on interface must be defined in classes which are implements interface.
3. We can inherit more than one interface on a class using comma(,) separator.
4. We can extend a interface on other interface.
5. We can declare constants inside a interface. Which will work as constants under class, except we cannot overwrite constants on other class or interface which implements or extends this interface.
6. Prior to PHP 5.3.9, a class could not implement two interfaces that specified a method with the same name, since it would cause ambiguity. More recent versions of PHP allow this as long as the duplicate methods have the same signature.

Final keyword in PHP

If we declare a function inside a class as final, we cannot use that function name in its derived classes. That is final keyword avoid method overriding. You can see more about method overriding here.

Example 1:
<?php
class A
{
     final function test()                // Declare function as final.
     {
          echo 'From base class';
     }
}

class B extends A
{
      function test()                      // Will throw an error.
      {
            echo 'This never run.';
      }
}
?>

And we can also avoid extending a class by declaring a class as final.

Example 2:
final class A                               // Declare class as final.
{
     function fun()
     {
     }
}

class B extends A                      // Will throw an error.
{
}

And note we can only declare function and class as final, cannot declare variable as final.

Constant variables in PHP

Basic Concept
By declaring a variable as constant, it will available all over the program and only we can access it, cannot modify it.

In PHP we can declare a constant variable in two ways. That are using define() function and const keywords.

Before going to see deep difference between these, first look a simple program using these two methods.

Example 1:
<?php
define('PIE',3.143);
const SHAPE='Circle';

function fun()
{
       echo PIE.'<br />';
       echo SHAPE;
}

fun();
?>

Output:
3.143
Circle
?>

On above example there no difference between using define() and const. But it has following notable differences.

1. const execute on compile time and define() execute on run time. That is we cannot declare const under a function or and conditional statement like "if".

2. We cannot declare define() in class, but can declare const.

See the below complex example which is using define() and const in all possible manners.

Example 2:
<?php
define('REGISTER_NO',90323540126);  // Declaring a constant using define keyword in global.
const NAME = 'Ramasamy Kasi';          // Declaring a constant using const keyword in global.
const ROLL_NO = 35;

function fun()
{
define('DEPARTMENT','Computer Science'); // Declaring a constant using define inside a funtion.
// const TOTAL = 450;    // Try to declare a constant using const inside a funtion. ITS WRONG.

echo REGISTER_NO.'<br />'; // Accessing consatants inside a function.
echo NAME.'<br />';
}

class Test
{
// define('AVERAGE',90) // Try to declare a constant using define inside a class. ITS WRONG.
const SEMESTER = 1;          // Declaring a constant using const inside a class.

function sample()
{
echo self::SEMESTER.'<br />';          // WAY 1: Accessing class constant.
}
}

fun(); // Calling function.
echo ROLL_NO.'<br />'; // Accessing constant, which is created in side a function.
echo DEPARTMENT.'<br />'; // Accessing constant, which is created on global.
$obj=new Test();
$obj->sample(); // Calling class function.
echo TEST::SEMESTER; // WAY 2: Accessing class constant.
?>

Output:
90323540126
Ramasamy Kasi
35
Computer Science
1
1

Static variables in PHP

Declaring a variable with a value inside function as static, is will not re-declare or reassign the value when every time that function will called. That is, the last assigned value will remains on that variable.

<?php
function test()
{
    static $a = 1;
    echo $a;
    if ($a <= 10) {
        ++$a;
        test();
    }
}

test(); // Will print: 1234567891011
test(); // Will print: 11
?>

Note:
1. We can not use any expression on declaring of static variables. Ex: static $a=5+6;  // Is wrong.
2. Declaring a variable as static on outside of function not make sense. Because it will not going to re-call in future, except in some cases like using goto statements.

Global variables in PHP

Basic Concept
By declaring a variable as Global(Common), we can access it any where of the program.

In C or C++
Here they don't use any special keyword to denote the variable as global. Say for example you defined  or declared a variable with a name in top of the program, now if you use the same name in any where of your program even inside a function, it will denote that as global variable.

Example 1:
#include<stdio.h>
// Global variables
int A=5;
int B=7;

int Add()
{
return A + B;
}

void main()
{
int answer; // Local variable
answer = Add();
printf("%d\n",answer);   // Output: 12
}

In PHP
Here to use global variable inside a function, first we have to declare it as "global".

Example 2:
<?php
$a = 1;
$b = 2;

function Sum()
{
    global $a, $b;
    $b = $a + $b;
}

Sum();
echo $b;   // Output: 3
?>

And one more way is we can use a super global valiable called $GLOBALS, intead of the keyword global.

Example 3:
<?php
$a = 1;
$b = 2;

function Sum()
{
    $GLOBALS['b'] = $GLOBALS['a'] + $GLOBALS['b'];
}

Sum();
echo $b;   // Output: 3
?>

We could not directly declare a class property as global variable. But we can get global variable under a function inside a class. So it can done by assigning global variable value on a property inside the constructor function.

Example 4:
<?php
$a=5;
class A
{
// private global $a; This is wrong statement. It will not going work.
        privare $a;

function __construct()
{
global $a;
$this->a=$a;
}

 function fun1()
{
global $a;
echo 'value of "a" inside class A is: '.$a;
}
}

$obj=new A();
$obj->fun1();
?>

References:
http://php.net/manual/en/language.variables.scope.php
http://www.codingunit.com/c-tutorial-functions-and-global-local-variables

Class inheritance in PHP

Definition

Creating a new class by inheriting(combining) exist class is called inheritance. By doing this newly created class can access existing classes functions and variables. Here newly created class will be called as sub class or derived class and the class from where it inherited is called as base class or super class.

Example

Books.php:

<?php

class Books // Class name.

{

var $title='5'; // Class variables.

var $price=45;

function __construct() // Constructor.

{

$this->price=100; // Define books's default price is 100.

}

function setTitle($title) // Set title for book.

{

$this->title=$title;

}

function getTitle() // Get tiitle of book.

{

return $this->title;

}

function setPrice($price) // Set price for book.

{

$this->price=$price;

}

function getPrice() // Get price of book.

{

return $this->price;

}

public function name() // A public function.

{

return '<br />Ramasamy Kasi : Public funtion from Books(base) class.';

}

protected function mobileNumber() // A protected function.

{

return '<br />+91 95684575000 : Protected funtion from Books(base) class.';

}

private function password() // A private function.

{

return '<br />bossword : Private funtion from Books(base) class.';

}

protected function getPassword() // Call private function from the same class(Books).

{

return $this->password();

}

function __destruct() // Destructor.

{

echo '<br />Destructor for "'.$this->title.'" Called.'; // Will be called when, there is no further action exist for a object.

}

}

?>

EBooks.php:

<?php

class EBooks extends Books         // Creating new class by combining existing class.

{

var $link;

function setLink($link) // Set link for book.

{

$this->link=$link;

}

function getLink() // Get link for book.

{

return $this->link;

}

function mobileNumber() // Getting mobile number by directly calling base classes protected function 'mobileNumber'.

{

$mob_from_base=Books::mobileNumber();

return $mob_from_base.' - Through EBooks(Sub) Class.';

}

function password() // Getting password by calling one base classes public function 'getPassword'.

{

$pass_from_base=$this->getPassword();

return $pass_from_base.' - Through EBooks(Sub) Class.';

}

}

?>

index.php:

<?php

include 'Books.php'; // Include Books class file.

include 'EBooks.php'; // Include EBooks class file.

$tamil=new Books(); // Creating object.

$tamil->setTitle('Tamil Text Book'); // Set tittle.

$tamil->setPrice(150); // Set price.

$english=new Books();

$english->setTitle('Spoken English');

$english->setPrice(180);

$environment=new Books();

$environment->setTitle('Environmental Science');

$python=new EBooks();

$python->setTitle('Python for Beginners');

$python->setLink('http://python.books/python_begin.html'); // Set link.

$python->setPrice(85);

echo $tamil->getTitle() . ' : ' . $tamil->getPrice().'<br />'; // Display saved title and price.

echo $english->getTitle() . ' : ' . $english->getPrice().'<br />';

echo $environment->getTitle() . ' : ' . $environment->getPrice().'<br />';

echo $python->getTitle() . ' : ' . $python->getPrice() . ' : ' . $python->getLink() . '<br />';

echo Books::name(); // Calling a public function of base class(Books).

echo EBooks::mobileNumber(); // Calling a protected function of base class(Books) by calling public function of sub class(EBooks).

echo $python->password().'<br />'; // Calling a private function of base class(Books) by calling public function of sub class(EBooks).

?>

Output

Tamil Text Book : 150
Spoken English : 180
Environmental Science : 100
Python for Beginners : 85 : http://python.books/python_begin.html

Ramasamy Kasi : Public funtion from Books(base) class.
+91 95684575000 : Protected funtion from Books(base) class. - Through EBooks(Sub) Class.
bossword : Private funtion from Books(base) class. - Through EBooks(Sub) Class.

Destructor for "Python for Beginners" Called.
Destructor for "Environmental Science" Called.
Destructor for "Spoken English" Called.
Destructor for "Tamil Text Book" Called.