The abstract class provides a partial implementation the other classes can build upon. If a class is declared as abstract using abstract keyword, it means that the class can contain incomplete methods that must be implemented in child classes, in addition to normal class members.

defined using the abstract keyword

1) An abstract class is a class that contains at least one abstract method.

2) Abstract Class object can not create.

3) In An abstract class a member variable never be an abstract, method only made abstract. 

 Example:

<?php 
abstract class parentClass
{
	public $name;
	abstract protected function calc($a, $b);
}
class childClass extends parentClass
{
	public function	calc($c, $d)
	{
		echo $c + $d;
	}
}
$test = new childClass();  // you can not create object of abstract class
$test->calc(10, 20);
?>

Output:

30