the documentation doesn't go too far in explaining the crucial difference between the two ways of declaring constants in PHP.
Const is handled at compile time, define() at run time. For this reason, a constant cannot be conditionally defined using Const, for example.
Another difference we can notice occurs in the constant declarations in classes. Const infiltrates the class scope, while define() leaks into the global scope.
<?php
Class Myclass (){
const NAME = "Nicolas";
}
?>
The NAME constant is within the scope of the MyClass class.