Constants in Classes

Part of the Object Oriented Programming section of Coddy's PHP journey — lesson 16 of 91.

Sometimes you need values in a class that should never change. Class constants provide a way to define fixed values that remain the same across all instances and cannot be modified after declaration.

You define a constant using the const keyword inside a class. By convention, constant names are written in uppercase:

<?php
class Circle {
    const PI = 3.14159;
    
    public $radius;
    
    public function __construct($radius) {
        $this->radius = $radius;
    }
    
    public function area() {
        return self::PI * $this->radius * $this->radius;
    }
}

$circle = new Circle(5);
echo $circle->area();

Output:

78.53975

Like static properties, constants belong to the class rather than individual objects. You access them using self::CONSTANT_NAME inside the class or ClassName::CONSTANT_NAME from outside:

<?php
class HttpStatus {
    const OK = 200;
    const NOT_FOUND = 404;
    const SERVER_ERROR = 500;
}

echo HttpStatus::OK . "\n";
echo HttpStatus::NOT_FOUND;

Output:

200
404

Key Point: Unlike static properties, constants cannot be changed once defined. They're ideal for values that should remain fixed throughout your application, such as configuration values, status codes, or mathematical constants.

Challenge

Easy

Let's build a temperature conversion utility that uses class constants to store fixed conversion values that never change.

You'll create two files to organize your code:

Temperature.php — Define a Temperature class that handles temperature conversions. The class should have three constants: FREEZING_POINT_C set to 0, BOILING_POINT_C set to 100, and CONVERSION_FACTOR set to 1.8 (used for Celsius to Fahrenheit conversion). Include a public $celsius property and a constructor that accepts and sets the Celsius value. Add a toFahrenheit() method that converts the stored Celsius temperature to Fahrenheit using the formula: (celsius * CONVERSION_FACTOR) + 32. Also add a getStatus() method that returns "Freezing" if the temperature equals the freezing point constant, "Boiling" if it equals the boiling point constant, or "Normal" otherwise.
main.php — Include the Temperature class file. Create three Temperature objects with values 0, 100, and 25. For each temperature, print the Fahrenheit conversion followed by the status on the same line in this format: "[fahrenheit]F - [status]". Each temperature should be on its own line. After printing all three, access the BOILING_POINT_C constant directly from the class (using Temperature::BOILING_POINT_C) and print it on a new line.

Remember that constants are accessed using self::CONSTANT_NAME inside the class and ClassName::CONSTANT_NAME from outside. Unlike properties, constants don't use the $ symbol when accessing them.

Cheat sheet

Class constants are fixed values that never change and are shared across all instances of a class. Define them using the const keyword:

<?php
class Circle {
    const PI = 3.14159;
    
    public function area($radius) {
        return self::PI * $radius * $radius;
    }
}

Access constants using self::CONSTANT_NAME inside the class:

return self::PI * $radius * $radius;

Access constants using ClassName::CONSTANT_NAME from outside the class:

echo Circle::PI;

Key differences from static properties:

Constants cannot be changed after declaration
Constants don't use the $ symbol
Constants are ideal for fixed values like configuration settings, status codes, or mathematical constants

By convention, constant names are written in uppercase:

<?php
class HttpStatus {
    const OK = 200;
    const NOT_FOUND = 404;
    const SERVER_ERROR = 500;
}

echo HttpStatus::OK;  // 200

Try it yourself

<?php

require_once 'Temperature.php';

// TODO: Create three Temperature objects with values 0, 100, and 25

// TODO: For each temperature, print in format: "[fahrenheit]F - [status]"
// Each temperature on its own line

// TODO: Access and print BOILING_POINT_C constant directly from the class
// Use ClassName::CONSTANT_NAME syntax

?>

Test yourself

This lesson includes a short quiz. Start the lesson to answer it and track your progress.

Constants in Classes

Cheat sheet

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