为什么未定义的常量值为true？

Question

Stupid question - I'm surprised this one has bitten me. Why do undefined constants in PHP evaluate to true?

Test case:

<?php
    if(WHATEVER_THIS_ISNT_DEFINED)
        echo 'Huh?';
?>

The above example prints 'Huh?'

Thanks so much for your help! :)

Answer 1

Try defined('WHATEVER_THIS_ISNT_DEFINED')

When PHP encounters a constant that is not defined, it throws an E_NOTICE, and uses the constant name you've tried to use as a string. That's why your snippet prints Huh!, because a non-empty string (which is not "0") will evaluate to true.

From the manual:

If you use an undefined constant, PHP assumes that you mean the name of the constant itself, just as if you called it as a string (CONSTANT vs "CONSTANT"). An error of level E_NOTICE will be issued when this happens.

If you set your error reporting level to report E_NOTICEs, which is a good practice during development, you will also see the notice thrown.

• PHP Constant Syntax
• defined()
• Casting to Boolean
• error_reporting
• error_reporting() function