http://3v4l.org/8knru

I didn't see a "hello world" outputted

Maybe the bug only exists on windows.
I got the error for 5.4.7 and 5.3.5

Tested on PHP 5.3.5 on Windows and I just get NULL as output all four times, and then the notice. Seems normal.

strange that you do not have the same problem. However, a notice should already be shown for the first usage of define.

please try again with a pure PHP (without any extension which will alter PHP 
behavior, like xdebug, xhprof, apc/opcache, or taint)

thanks

I still got the "hello world" even without any extension. The output is slightly different, I got NULL instead of null without xdebug. But still 
NULL
NULL
NULL
string(11) "hello world"
Notice: Constant null already defined

for PHP 5.3.5 and PHP 5.4.7

In my opinion this bug can be closed because probably no one will ever do this. 
As side mark, also http://3v4l.org/8knru does not show a notice for the first define. Thus the implementation is not entirely correct. I assume your PHP version behaves differently than mine in terms of symbol resolution. In my PHP version the case matters. So if I write NULL a different symbol is resolved than when I write null. In your version always null is resolved. That could be the reason why you do not get "hello world" as output. But I do not really know the internals of PHP, it's just an assumption.

For some reason my earlier comment was not published here.

It contained a table of test results from 7 different PHP versions and 3 testing modes. I'll have no time to rebuild the table now, but the point is that I confirmed the bug on Linux for various PHP versions (including trunk snapshot). However, the bug appeared only in __interactive__ CLI mode, not in `php -f` or PHP run from Apache. It seems that any NULL spelling is affected, except all-lowercase version. Also redefinition of particular spelling doesn't affect other spellings, so one can define NuLl as 'x', nULL as 'y' and each of them will keep their defined value.

I would also point out that a link provided by Laruence in the first comment outputs "hello world" for some versions.

The same issue applies to TRUE and FALSE.

Example:
define('TRUE', FALSE);
define('FALSE', TRUE);
var_dump(TRUE);
var_dump(FALSE);

Some more information about the define statement. I can also use define for things like that:
define("1",0);
define("if","else");

It doesn't have any influence on the code, if one writes 1 or if after the define then 1 is still an int and if still the keyword if, but it should not be possible in my opinion.

I've just noticed somene's code on 3v4l: a lowercase versions may be redefined too if they're used in namespace.
---------------------------------
namespace ns;
define('ns\\true', 'blarg1');
define('ns\\false', 'blarg2');
define('ns\\null', 'blarg3');
var_dump(true, false, null);
---------------------------------
This also works in PHP run from Apache and `php -f` (tested in 5.4.14 on Arch64).

To make things funnier:
---------------------------------
define('int', 123);
var_dump(int + int);
---------------------------------

No feedback was provided. The bug is being suspended because
we assume that you are no longer experiencing the problem.
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information that was requested earlier, please do so and
change the status of the bug back to "Re-Opened". Thank you.