为什么PHP var_export（）打印NULL大写，但是true和false小写？

Question

See https://3v4l.org/6Oelk

var_export(array(NULL, TRUE, FALSE));

Result:

array (
  0 => NULL,
  1 => true,
  2 => false,
)

NULL is uppercase, but true and false are lowercase.

I suppose this has a historic reason, but I don't find it documented anywhere.

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The following experiment is also quite interesting, and suggests that internally the "canonical" spelling of null, true and false is lowercase, contrary to what var_export() does: https://3v4l.org/ggM4E

define('false', '- false -');
define('FALSE', '- FALSE -');
define('null', '- null -');
define('NULL', '- NULL -');
var_export(array(TRUE, true, FALSE, false, NULL, null));

It shows that:

• define('NULL', ..) and define('FALSE', ..) are allowed but have no effect in recent PHP versions.
• define('null', ..) and define('false', ..) give a notice, but also have no effect in recent PHP versions.
• In older PHP version (4.x), 'NULL' and 'FALSE' CAN be redefined and it does have an effect. But 'null' and 'false' cannot be redefined.

Oh, just for completeness: define('NULL', ..) and define('FALSE', ..) are actually NOT ignored if within a namespace, in PHP 5.x. See https://3v4l.org/JusvB. Or if you really want, you can even override 'null' and 'false' within a namespace: https://3v4l.org/i796C

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Related: Uppercase Booleans vs. Lowercase in PHP, see this answer: https://stackoverflow.com/a/3807178/246724

Answer 1

As @NikiC notes in the OP comments, there's no specific reasoning for this.

The most logical explanation though would be that considering PHP is written in C and historical standards define NULL as uppercase, and true, false as lowercase, reason follows that the standards in PHP would follow suit.

IEEE Std 1003.1 References:

• stdbool - http://pubs.opengroup.org/onlinepubs/009695399/basedefs/stdbool.h.html
• stddef - http://pubs.opengroup.org/onlinepubs/7908799/xsh/stddef.h.html