Class constants are a bit different than normal(?) constants. Mostly they are used to declare some constant data inside a class.
You can define class constants inside a class with a const keyword and they’ll stay unchangeable. They can be redefined by the child class though.
Constant identifiers are case-sensitive but they are always defined in uppercase.
By default, the visibility of class constants if public.
You can have constants in interfaces too!
You can reference a class using a variable but the value of the variable can not be a keyword like self, static or parent.
Class constants are allocated once per class, and not for each class instance.
Class Constant Examples
You can use the scope resolution operator (::) to access the class constant. For e.g.
Class_name::CONSTANT
Class constants can be simple like this:
<?php
class Greetings {
const MESSAGE = "Hello World!";
}
echo Greetings::MESSAGE; //Hello World!
?>
You can also define it inside a method:
<?php
class Greetings {
const MESSAGE = "Hello World!";
function showGreetings() {
echo self::MESSAGE;
}
}
$g = new Greetings();
$g->showGreetings(); //Hello World!
?>
Class constant expression example
<?php
const ONE = 1;
class foo {
const TWO = ONE * 2;
const THREE = ONE + self::TWO;
const SENTENCE = 'The value of THREE is '.self::THREE;
}
echo foo::SENTENCE;
//The value of THREE is 3
?>
Class constants with visibility
From PHP 7.1.0, you can define class constants with visibility (access) modifiers.
<?php
class Foo {
public const BAR = 'bar';
private const BAZ = 'baz';
}
echo Foo::BAR; //bar
echo Foo::BAZ; //Fatal error
?>
class constant examples object-oriented OOP
