The list of reserved words in PHP includes the “abstract” keyword. When a class is defined with the “abstract” keyword, it cannot be instantiated, i.e., you cannot declare a new object of such a class. An abstract class can be extended by another class.

abstract class myclass {
   // class body
}

As mentioned above, you cannot declare an object of this class. Hence, the following statement −

$obj = new myclass;

will result in an error message as shown below −

PHP Fatal error:  Uncaught Error: Cannot instantiate abstract class myclass

An abstract class may include properties, constants or methods. The class members may be of public, private or protected type. One or more methods in a class may also be defined as abstract.

If any method in a class is abstract, the class itself must be an abstract class. In other words, a normal class cannot have an abstract method defined in it.

This will raise an error −

class myclass {
   abstract function myabsmethod($arg1, $arg2);
   function mymethod() #this is a normal method {
      echo "Hello";
   }
}

The error message will be shown as −

PHP Fatal error:  Class myclass contains 1 abstract method 
and must therefore be declared abstract

You can use an abstract class as a parent and extend it with a child class. However, the child class must provide concrete implementation of each of the abstract methods in the parent class, otherwise an error will be encountered.

Example

In the following code, myclass is an abstract class with myabsmethod() as an abstract method. Its derived class is mynewclass, but it doesn’t have the implementation of the abstract method in its parent.

<?php
   abstract class myclass {
      abstract function myabsmethod($arg1, $arg2);
      function mymethod() {
         echo "Hello";
      }
   }
   class newclass extends myclass {
      function newmethod() {
         echo "World";
      }
   }
   $m1 = new newclass;
   $m1->mymethod();
?>

The error message in such a situation is −

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

It indicates that newclass should either implement the abstract method or it should be declared as an abstract class.

Example

In the following PHP script, we have marks as an abstract class with percent() being an abstract method in it. Another student class extends the marks class and implements its percent() method.

<?php
   abstract class marks {
      protected int $m1, $m2, $m3;
      abstract public function percent(): float;
   }

   class student extends marks {
      public function __construct($x, $y, $z) {
         $this->m1 = $x;
         $this->m2 = $y;
         $this->m3 = $z;
      }
      public function percent(): float {
         return ($this->m1+$this->m2+$this->m3)*100/300;
      }
   }

   $s1 = new student(50, 60, 70);
   echo "Percentage of marks: ". $s1->percent() . PHP_EOL;
?>

It will produce the following output −

Percentage of marks: 60

Difference between Interface and Abstract Class in PHP

The concept of abstract class in PHP is very similar to interface. However, there are a couple of differences between an interface and an abstract class.