How to create a class constant

class Person {
  const GREEN_EYES = 1;
  const BLUE_EYES = 2;
  const BROWN_EYES = 3;
  private $eye_color;
  public function getEyeColor() {
    return $this->eye_color;
  } 
  public function setEyeColor($value) {
    if ($value == self::GREEN_EYES || $value == self::BLUE_EYES || $value == self::BROWN_EYES) {
$this->eye_color = $value;
    } else {
exit('Eye Color Not Set');
    }
  }
}

$person = new Person();
$person->setEyeColor(Person::GREEN_EYES);

Description

• A class constant is a constant value that belongs to the class, not to objects created from the class.
• To access a constant that belongs to a class, you can code the name of the class followed by a double colon. This double colon is known as the scope resolut operator. It is also known as the Paamayim Nekudotayim, which means double colon in Hebrew.
• Inside a class, you can access a class constant by coding the self keyword followed by a double colon and the class constant name.
• Outside the class, you can access a class constant by coding the class name followed by a double colon and the class constant name.
• Class constants are always public. You can’t use the public, private, or protected keywords with them.
• Class constants are typically used for defining a set of options that are passed to methods in the class.