Modifier

A modifier is a keyword that changes the behavior or properties of a class element, such as property, method, or constant. PHP supports several categories of modifiers:

Visibility modifiers control access to class members:

• public: accessible from anywhere, by default

• protected: accessible within the class and its subclasses

• private: accessible only within the declaring class

Other modifiers:

• static: belongs to the class itself rather than instances

• abstract: declares a class or method that must be implemented by subclasses

• final: prevents a class from being extended or a method from being overridden

• readonly: makes a property immutable after initialization

• readonly class: makes all properties readonly

Modifiers are essential for implementing encapsulation, polymorphism, and other OOP principles. They enforce design constraints at the language level, improving code safety and clarity.

<?php

    abstract class Shape {
        protected float $area;

        abstract public function calculate(): float;

        final public function describe(): string {
            return 'This shape has an area of ' . $this->calculate();
        }
    }

    class Circle extends Shape {
        public function __construct(
            private readonly float $radius
        ) {}

        public function calculate(): float {
            return pi() * $this->radius  2;
        }
    }

    $c = new Circle(5);
    echo $c->describe();

?>

Documentation

See also PHP Visibility and PHP Class Keywords.

Related : Visibility, Private Visibility, Protected Visibility, Public Visibility, static, Abstract Keyword, Final Keyword, Readonly, Data Hiding, Encapsulation, OOP (Object Oriented Programming), Properties, Method, Static Constant