PHP - assign value of function to class constant

Question

In PHP when you define a class level constant as in:

const MY_CONSTANT = 'hello constant';

Why can't you initialize this value with a function such as

const MY_FILEPATH = dirname(dirname(__FILE__)) . '/heres-my-file.php';

Answer 1

In short: The constants are replaced while parsing, but functions are executed while interpreting. The parser simply cannot know to what it should set the value of the content.