PHP : OOP - Class Constants

In PHP, class constants are similar to regular constants but are defined within a class. Class constants are accessed using the `::` scope resolution operator and are useful for defining values that should not change throughout the execution of a script. They are declared using the `const` keyword inside a class and are accessed without using the `$` prefix.

Syntax:


class ClassName {
    const CONSTANT_NAME = value;
}
 

Example of Class Constants:

Here's an example to illustrate how class constants work in PHP:


class Math {
    const PI = 3.14159;
    const EULER = 2.71828;

    public static function areaOfCircle($radius) {
        return self::PI * $radius * $radius;
    }
}

// Accessing class constants
echo "Value of PI: " . Math::PI . "<br>";      // Output: Value of PI: 3.14159
echo "Value of EULER: " . Math::EULER . "<br>"; // Output: Value of EULER: 2.71828

// Using a class constant in a method
$radius = 5;
$area = Math::areaOfCircle($radius);
echo "Area of circle with radius {$radius}: {$area}<br>"; // Output: Area of circle with radius 5: 78.53975
 

In this example:

  • The Math class defines two class constants PI and EULER.
  • These constants are accessed using the Math:: syntax without needing to instantiate the class.
  • The areaOfCircle() method uses the self::PI class constant to calculate the area of a circle.

Visibility of Class Constants:

Class constants can have different visibility levels:

  • Public: The default visibility. Can be accessed from outside the class.
  • Protected: Can only be accessed within the class itself and its subclasses.
  • Private: Cannot be accessed from outside the class.

Example with Different Visibility:


class MyClass {
    public const PUBLIC_CONST = "Public Constant";
    protected const PROTECTED_CONST = "Protected Constant";
    private const PRIVATE_CONST = "Private Constant";

    public function displayConstants() {
        echo self::PUBLIC_CONST . "<br>";     // Accessible within the class
        echo self::PROTECTED_CONST . "<br>";  // Accessible within the class
        echo self::PRIVATE_CONST . "<br>";    // Accessible within the class
    }
}

// Accessing class constants
echo MyClass::PUBLIC_CONST . "<br>";        // Output: Public Constant
// echo MyClass::PROTECTED_CONST . "<br>";  // Error: Cannot access protected constant
// echo MyClass::PRIVATE_CONST . "<br>";    // Error: Cannot access private constant

$obj = new MyClass();
$obj->displayConstants(); // Output: Public Constant, Protected Constant, Private Constant
 

In this example:

  • MyClass has three class constants with different visibility levels.
  • Inside the displayConstants() method, all three constants are accessible because they are within the same class.

Naming Convention for Class Constants:

  • Class constants are typically in uppercase with words separated by underscores ( _ ).
  • This is not a strict rule, but it's a common convention to distinguish constants from regular variables.

Conclusion:

Class constants in PHP are defined using the const keyword within a class and provide a way to define values that do not change during script execution. They are accessed using the :: scope resolution operator and are useful for defining values that are associated with a class. Class constants can have different visibility levels and are a useful tool in object-oriented PHP programming for defining and using constants within a class.