Constantes PHP

Configuracao.php

<?php

define('FOO','Hello World');`

Foo.php

<?php

namespace testeConstant;

Class Foo {
    public function exibeFoo(){
        echo \FOO; // Hello World
    }
}

index.php

<?php

include_once("Configuracao.php");
require_once('Foo.php');

use testeConstant\Foo;

$foo = new Foo();

$foo->exibeFoo();