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

abstract_class-science.php

<?php
abstract class Physics {
    /**
     * The Planck's law
     * @param integer       $J          An integer that represent an energy measure
     * @param integer       $s          An integer that represent a time measure
     */
    public static function Planck($J, $s) {
        $h = 6.62607004 * pow(10, -34) * ($J * $s);
        return $h;
    }

    /**
     * The Quantum Mechanics method
     * @param string        $type       Is this "deterministic"?
     * @param string        $elements   Some definition elements
     */
    public static function QuatumMechanics($type, $elements) {
        if($type == "deterministic") {
            return "No randomness and " . $elements;
        }
    }

    /**
     * The Bohm Theory, pure abstract definition
     */
    abstract public static function BohmTheory();
}

/**
 * The class that extends the above abstract
 */
class PhysicsTheories extends Physics {
    /**
     * Bohm Theory
     * @return string                   Quantum mechanics data
     */
    public static function BohmTheory() {
        return Physics::QuatumMechanics("deterministic", "non-local Universe") . ".\nUses also the Plack's law, which if J = 10 and s = 2 is the equivalent of " . Physics::Planck(10, 2);
    }
}
?>