subtle PHP 4 to PHP 5 difference regarding objects

UPDATE:Some internal classes define their own cast handlers, such as simplexml. So, if you cast internal objects to bool, the result depends on the particular object. simplexml objects always cast their internal value to string prior to casting to other values, so an empty tag will cast to false. The actual source is viewable at http://lxr.php.net/source/php-src/ext/simplexml/simplexml.c#1539

Recently, some code in the PEAR installer was discovered to be invalid in PHP 4. After a bit of investigation, I realized that the significant difference in the way objects are represented internally in PHP 5 was the culprit. In essence, the difference shows itself in this code:

In PHP 5, this displays as bool(true), but in PHP 4, it displays bool(false). The reason is that in PHP 4, objects are simply glorified associative arrays, and so PHP treats the above code exactly the same as if it were:

As we know, an empty array evaluates to false in both PHP 4 and PHP 5. Note that modifying the code above even slightly like so:

removes this difference. Why PHP 4 then treats the code as if it were:

This results in true, as the array is no longer empty.

In short, if you're writing code that you expect to work in both PHP 4 and PHP 5 that tests for the type of a variable, always use the is_object() function, and never test with a boolean conversion as in:

[geshi lang=php]