Abstract Classes in PHP

<?php 
abstract class Base 
{ 
    abstract function printdata(); 
    function pr() 
    { 
        echo "Base class"; 
    } 
} 

class Derived extends Base 
{ 
    function printdata() { 
        echo "Derived class"; 
    } 
} 

// Creating an object of Derived class
$b1 = new Derived; 
$b1->printdata(); 
?>

<?php
abstract class Base {
    abstract function printdata();
}
class Derived extends base {
    function printdata() {
        echo "Derived class";
    }
}

// Uncommenting the following line will cause a fatal error:
// $b = new Base();   
    
$b1 = new Derived;
$b1->printdata();
?>

<?php 

abstract class Base { 
    function __construct() { 
        echo "This is an abstract class constructor"; 
    } 

    abstract function printdata(); 
} 

class Derived extends Base { 
    function __construct() { 
        echo "\nDerived class constructor"; 
    } 
    
    function printdata() { 
        echo "\nDerived class printdata function"; 
    } 
} 

$b1 = new Derived; 
$b1->printdata(); 

?>

<?php
abstract class Base {
    abstract function printdata() {
        echo "Parent class printdata";
    }
}
?>

<?php
abstract class Base { 
    abstract function printdata(); 
}
?>