Greetings! In today's lesson, we'll unravel the concept of polymorphism in PHP's Object-Oriented Programming (OOP). Grasping polymorphism enables us to use a single entity (a method or class) to represent different types in various scenarios. Let's proceed.

Polymorphism, a pillar of OOP, allows one object to embody multiple forms. Visualize a button in software; depending on its type (for instance, a submit button or a radio button), the action resulting from pressing it varies. This dynamic encapsulates the spirit of polymorphism!

PHP supports polymorphism mainly through dynamic or run-time polymorphism, leveraging method overriding within abstract classes. Let's observe dynamic polymorphism in action within a simple application involving shapes. The base Shape class has an area method, which calculates the area for shapes. This method is uniquely implemented in the subclasses Rectangle and Circle.

<?php

abstract class Shape {
    // Abstract method for calculating area
    abstract public function area();
}

class Rectangle extends Shape {
    private $length;
    private $width;

    public function __construct($length, $width) {
        $this->length = $length;
        $this->width = $width;
    }

    // Override the abstract method to calculate rectangle area
    public function area() {
        return $this->length * $this->width;
    }
}

class Circle extends Shape {
    private $radius;

    public function __construct($radius) {
        $this->radius = $radius;
    }

    // Override the abstract method to calculate circle area
    public function area() {
        return pi() * $this->radius * $this->radius;
    }
}

$rectangle = new Rectangle(2, 3);
echo $rectangle->area() . "\n"; // Prints: 6

$circle = new Circle(5);
echo $circle->area() . "\n"; // Prints: 78.539816339745

Here, polymorphism shines as the area() method takes on multiple forms while using the same Shape abstract class. It behaves differently depending on whether the object is a Rectangle or a Circle.