• A constant is an identifier (name) for a simple value. As the name suggests, that value cannot change during the execution of the script.
  • Do not use the define() method for declaring a constant as it does not work within classes.
  • Constants defined in a class are READ-ONLY and STATIC (shared by all instances of the class).
  • Constants Manual (
// Define a constant.
const MY_CONSTANT = 'a value';

// Use a constant.
echo MY_CONSTANT; // Prints "a value."

// Use with a class.
class Foo {

    const MY_CONST = 'bar';

    // Use a constant in a method.
    public function get_my_const() {
        return self::MY_CONST;

    // Call a constant dynamically.
    public function get_constant($name) {
        return constant("self::{$name}");

// Use statically.
echo Foo::MY_CONST; // prints 'bar'

// Use with an instantiated class.
$foo = new Foo();
echo $foo->get_my_const(); // prints 'bar'
echo $foo->get_constant('MY_CONST'); // prints 'bar'
echo $foo::MY_CONST; // prints 'bar'