PHP Class Constant

A PHP class constant is a fixed value that remains the same throughout the lifetime of a class. Constants are defined using the const keyword and are typically used to define immutable values that are shared across all instances of a class. Constants are accessed using the scope resolution operator ::.

Here’s a basic example to illustrate PHP class constants:

Example 1: Defining and Accessing Class Constants

<?php
class MyClass {
    const MY_CONSTANT = 'Hello, World!';
    
    public function showConstant() {
        echo self::MY_CONSTANT;
    }
}

// Accessing the constant from outside the class
echo MyClass::MY_CONSTANT;  // Output: Hello, World!

// Accessing the constant from inside the class
$obj = new MyClass();
$obj->showConstant();  // Output: Hello, World!
?>

In this example:

• MY_CONSTANT is defined within MyClass.
• The constant is accessed from outside the class using MyClass::MY_CONSTANT.
• The constant is accessed from inside the class using self::MY_CONSTANT.

Example 2: Constants in Inheritance

When a class is extended, the child class can access the constants of the parent class.

<?php
class ParentClass {
    const PARENT_CONSTANT = 'Parent Constant';
}

class ChildClass extends ParentClass {
    const CHILD_CONSTANT = 'Child Constant';
    
    public function showConstants() {
        echo self::CHILD_CONSTANT . "\n";
        echo parent::PARENT_CONSTANT . "\n";
    }
}

// Accessing the constants from outside the class
echo ChildClass::CHILD_CONSTANT;   // Output: Child Constant
echo ChildClass::PARENT_CONSTANT;  // Output: Parent Constant

// Accessing the constants from inside the class
$child = new ChildClass();
$child->showConstants();  
// Output:
// Child Constant
// Parent Constant
?>

In this example:

• ParentClass has a constant PARENT_CONSTANT.
• ChildClass extends ParentClass and adds its own constant CHILD_CONSTANT.
• The child class can access its own constants using self::CONSTANT_NAME and parent class constants using parent::CONSTANT_NAME.

Example 3: Using Constants in Static Methods

Constants can also be used in static methods within the class.

<?php
class MyClass {
    const MY_CONSTANT = 'Static Constant';
    
    public static function showStaticConstant() {
        echo self::MY_CONSTANT;
    }
}

// Accessing the static method from outside the class
MyClass::showStaticConstant();  // Output: Static Constant
?>

In this example:

• MY_CONSTANT is accessed within the static method showStaticConstant using self::MY_CONSTANT.
• The static method showStaticConstant is called using MyClass::showStaticConstant.

These examples demonstrate how to define and use class constants in PHP, including accessing them from within the class, from outside the class, in child classes, and in static methods.