7.7 PHP Class Constants

• Constants cannot be changed once it is declared.
• Class constants can be useful if you need to define some constant data within a class.
• A class constant is declared inside a class with the const keyword.
• Class constants are case-sensitive. However, it is recommended to name the constants in all uppercase letters.
• We can access a constant from outside the class by using the class name followed by the scope resolution operator (::) followed by the constant name, like here:

Example 1: PHP Class Constants part 1

<?php
class Goodbye {
  const LEAVING_MESSAGE = "Thank you for visiting W3Schools.com!";
}

echo Goodbye::LEAVING_MESSAGE;
?>

Or, we can access a constant from inside the class by using the self keyword followed by the scope resolution operator (::) followed by the constant name, like here:

Example 2: PHP Class Constants part 2

<?php
class Goodbye {
  const LEAVING_MESSAGE = "Thank you for visiting W3Schools.com!";
  public function byebye() {
    echo self::LEAVING_MESSAGE;
  }
}

$goodbye = new Goodbye();
$goodbye->byebye();
?>