PHP OOP – Class Constants

In PHP OOP, class constants are similar to class properties, but they are immutable and their values cannot be changed once they are defined. Class constants are defined using the const keyword and are accessed using the :: operator.

Here’s an example of a class with a constant:

class Circle {
    const PI = 3.14;
    public $radius;
    public function __construct($radius) {
        $this->radius = $radius;
    public function area() {
        return self::PI * pow($this->radius, 2);
$circle = new Circle(5);
echo Circle::PI; // Output: 3.14
echo $circle->area(); // Output: 78.5

In this example, the Circle class has a constant named PI that is set to the value 3.14. The class also has a property named $radius and a method named area() that calculates the area of a circle using the formula pi * radius^2. The area() method uses the self::PI syntax to access the value of the PI constant.

To access the value of the constant, we use the :: operator:

echo Circle::PI; // Output: 3.14

We can also use the constant in a class method, as shown in the area() method of the Circle class.

In summary, class constants in PHP OOP are immutable values that are defined using the const keyword. They are accessed using the :: operator and can be used in class methods.