Abstract Method

An abstract method is a method that has no code. It also use the abstract option in the syntax.

An abstract method serves as a blueprint for other methods: it host class is intended to be subclassed.

Interfaces only have abstract methods.

Abstract constructors cannot use promoted properties.

<?php

abstract class AbstractClass {
    abstract function foo(); // abstract method

    function goo() {
        return 1;
    } // concrete method
}

class ConcreteClass extends AbstractClass{

    function foo() {
        return 2;
    } // concrete method

    // No need to redefine the concrete methods.
    // Yet, it is still possible
}

?>

Documentation

See also PHP: Abstract Classes, Interfaces, and Traits Explained.

Related : Final Keyword, Concrete Class, Concrete Method, Abstract Class