Code sample for the blog post: http://iod.io/Post/4/Abstract_classes_and_Interfaces-a_clear_explanation

interface-philosophy.php

<?php
interface PhilosophicalContexts {
    /**
     * The concept of Property
     * @see http://www.iep.utm.edu/prop-con/
     */
    public function Property();
}
    interface PhilosophicalTheories extends PhilosophicalContexts {
        /**
         * The John Locke's Labour theory of property
         * @see https://en.wikipedia.org/wiki/John_Locke
         */
        public function LabourTheory();

        /**
         * The Adam Smith's theory of "invisible hand"
         */
        public function InvisibleHand();
    }

/**
 * Now the class that implements above Interfaces
 */
class Economics implements PhilosophicalTheories {
    /**
     * The concept of Property
     */
    public function Property() {
        $this->context = "Economics";
        $this->definition = "The ownership of goods";
        return $this;
    }

    /**
     * The John Locke's Labour theory of property
     */
    public function LabourTheory() {
        $this->author = "John Locke";
        $this->definition = "God granted dominion over nature, and the man acquires the property with its work (by the relationship with the manipuled part of nature)";
        return $this;
    }
    
    /**
     * The Adam Smith's Invisible Hand theory of property
     */
    public function InvisibleHand() {
        $this->author = "Adam Smith";
        $this->definition = "All the economics properties are given by God, and supply and demand are 'led by an invisible hand'";
        return $this;
    }
}
?>