undeclared attributes in PHP classes

I was recently surprised to discover that a PHP object can have attributes (variables) not declared in the class. The following works in PHP4 and PHP5 (it prints 'ick' and then 'yark'):

class Gakkk {}

$gakkk = new Gakkk();
$gakkk->blech = 'ick';
echo $gakkk->blech, "\n";
$gakkk->blech = 'yark';
echo $gakkk->blech, "\n";

At best, this strikes me as a very poor programming practice. Who knows when this sort of thing will stop working (in a future version of PHP)? And what kind of code readability is this?

I really like PHP, but this is just weird to me.