PHP - '=' Alignment.

I'm using PHPStorm 2.1.4 on OSX 10.7, and when I go to Preferences > Code Style > Wrapping & Braces > PHP, I can't check Assignment statement > Align when multiline.
Why is this? It's just a disabled checkbox.

I'd really like to enable this, so my:

$a = 10;
$ab = 10;

Can become:

$a  = 10;
$ab = 10;

Thanks!

- Josh