Polymorphism Principle

Polymorphism is one of the 4 main OOP principles. It's the ability to take many forms.

Runtime Polymorphism

Runtime Polymorphism occurs in function overriding .

This means that the base class has a function that's being overwritten in the children classes:

<?php
class Figure
{
    // base class method that'll be overwritten:
    public function calcArea()
    {
        return 0;
    }
}

class Square extends Figure
{
    private $side;

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

// overwriting base method:
    public function calcArea()
    {
        return $this->side * $this->side;
    }
}

class Triangle extends Figure
{
    private $a;
    private $b;
    private $c;

public function __construct($aVal, $bVal, $cVal)
    {
        $this->a = $aVal;
        $this->b = $bVal;
        $this->c = $cVal;
    }

// overwriting base method:
    public function calcArea()
    {
        $a = $this->a;
        $b = $this->b;
        $c = $this->c;

$p = ($a + $b + $c) / 2;
        $area = sqrt($p * ($p - $a) * ($p - $b) * ($p - $c));

return $area;
    }
}

$square = new Square(2);
echo "Square Area = " . $square->calcArea() . "\n";

$triangle = new Triangle(3, 4, 5);
echo "Triangle Area = " . $triangle->calcArea() . "\n";

Output:

The function calcArea in Figure class is overwritten by Square and Triangle classes.

Unlike statically typed languages (C++, Java, C# and others), there is no such thing as compile-time polymorphism in PHP.