I do not know much about how $[‘globals’] decides what browser you are using, i assume it uses some kind of browser sniffing which is generally a bad technique as the results it produces can be less than reliable.
Why do you need to use PHP anyway, just putting this as HTML in the head will achieve exactly the same effect.
<!--[if lt IE 9]>
<script src="http://html5shim.googlecode.com/svn/trunk/html5.js"></script>
<![endif]-->