Constants are like variables except that once they are defined, they cannot be changed. If we need to define some constant data within a class, we need to use class constants. Class constants are case-sensitive and they can be declared inside a class with the const keyword. We can also access a constant outside of a class by using the class name followed by the :: operator with the constant name.

Example:

<?php
class Namaste {
  const GREETING_MSG = "Namaskaram Dosto!!";
}
 
echo Namaste::GREETING_MSG;
?>

We can also use the self keyword to access the constant inside the class.

Example:

<?php
class HelloWorld {
  const NEW_MESSAGE = "Did I leave the Stove on?";
  public function ByeWorld() {
    echo self::NEW_MESSAGE;
  }
}
 
$helloworld = new HelloWorld();
$helloworld->ByeWorld();
?>

Note: Corrected the constant name to follow the convention of using uppercase letters with underscores for readability.