PHPクラスの抽象化

<?php
class testclass{
   //
}
?>

$a=new testclass();
PHP Fatal error: Uncaught Error: Cannot instantiate abstract class testclass

<?php
abstract class testclass{
   abstract function test1();
   abstract function hello();
}
class myclass extends testclass{
   function test1(){
      echo "Overrides parent test method";
   }
}
$a=new myclass();
?>

PHP Fatal error: Class myclass contains 1 abstract method and must therefore be declared abstract or implement the remaining methods (testclass::hello)

<?php
abstract class testclass{
   abstract function hello($name, $age);
}
class myclass extends testclass{
   function hello($name, $age){
      echo "My name is $name and my age is $age";
   }
}
$a=new myclass();
$a->hello("Ravi",20);
?>