Thank you for taking the time to write to us, but this is not
a bug. Please double-check the documentation available at
http://www.php.net/manual/ and the instructions on how to report
a bug at http://bugs.php.net/how-to-report.php

You can only initialize class constants with constant values. You pass a statement which has to be evaluated at runtime, while the class constants are defined at compile time which comes earlier.

Thanks for the reply. I suspected this would be marked bogus, since the behavior is consistent with how object properties need to be initialized, etc.

Can this be considered a feature request? I do understand the runtime/compile time challenge. But it seems like there may be ways around it (e.g. late static binding is getting addressed).

Maybe a solution for this issue could be using defined constants, something like 
this:

[ START ]

define('ROOT_DIR', dirname('..' . Constants::DS));
define('ACTIONS_DIRECTORY', APPLICATION_PATH . Constants::DS . 'actions' . 
Constants::DS);
define('VIEWS_DIRECTORY', APPLICATION_PATH . Constants::DS . 'views' . 
Constants::DS);

class Constants {
	const ROOT_DIR = ROOT_DIR;
	const DS = DIRECTORY_SEPARATOR;
	const ACTIONS_DIRECTORY = ACTIONS_DIRECTORY;
	const VIEWS_DIRECTORY = VIEWS_DIRECTORY;
}

[ END ]

Sorry by my poor english, maybe someone could correct me...

Please re-check, the behaviour is inconsistent.

is inconsistent between global consts and class const.
And is inconsistent between class const definition itself.

[2007-08-21 06:22 UTC] derick@php.net said:

"You can only initialize class constants with constant values. You pass a statement which has to be evaluated at runtime, while the class constants are defined at compile time which comes earlier."

Then, it seems that something goes wrong, as I can't assign a class const to a statement but I can assign it to a const which has an statement... Which, by transitive properties :) I'm assigning an statement to it...

[ START ]

define('STATEMENT', 'hello' . 'world';

class Constants {
	const WORKS = STATEMENT; // works.
	const FAILS = 'hello' . 'world'; // fails.
}

[ END ]

Additional tests, try this, according to runtime/compile blah blah, should this work? Please raise these to a feature request.

<?php

define('HELLO_WORLD', Constants::HELLO . Constants::WORLD);

class Constants {
  const HELLO = 'Hello ';
  const WORLD = 'world.';
  const HELLO_WORLD = HELLO_WORLD;
}

echo Constants::HELLO_WORLD;

@sdeonline: This has been implemented for PHP 5.6, see https://wiki.php.net/rfc/const_scalar_exprs.