PHP undefined constant testing

if($FOO) do_something(); 

Just using FOO takes it as a value rather than the variable you defined. Better to use PHP's defined.

If you enable error logging, you'll see an error like the following:

PHP Notice: Use of undefined constant BOO - assumed 'BOO' in file at line N

What's happening is that PHP is just arbitrarily assuming that you meant to use 'BOO' and just forgot the quotes. And since strings other than '' and '0' are considered "true", the condition passes.

If it's not the existance of the constant you want to test, but if you want to test the value of the constant you defined, this might be a better way: if(BOO === true) or if(BOO === false)

// BOO has not been defined

if(BOO) do_something();

BOO will be coerced into the string BOO, which is not empty, so it is truthy.

This is why some people who don't know better access an array member with $something[a].

You should code with error_reporting(E_ALL) which will then give you...

Notice: Use of undefined constant HELLO - assumed 'HELLO' in /t.php on line 5

You can see if it is defined with defined(). A lot of people use the following line so a PHP file accessed outside of its environment won't run...

<?php defined('APP') OR die('No direct access');

This exploits short circuit evaluation - if the left hand side is true, then it doesn't need to run the right hand side.