php string syntax question with html

Hello,
I've got what is probably a very simple question, probably something
having to do with quotes single vs. double, but the answer is
frustrating elusive, I keep getting a syntax error.
I'm trying to customize a wordpress theme a friend sent me. We're both
using apache as web server and php5, but his has got to be configed
differently than mine. The theme deals with multiple stylesheet
inclusion among other things. The original line is:

$styleSheets[0]["sheet"]='<link
href="/wp-content/themes/theme/style/white.css" rel="stylesheet"
type="text/css" />';

That code puts the <link in the head portion of the document. The
issue is his / is not where mine is, i'm using a virtual host and need
a line similar to this:

$styleSheets[0]["sheet"]='<link href=$_SERVER['DOCUMENT_ROOT'] .
"/wp-content/themes/theme/style/white.css" rel="stylesheet"
type="text/css" />';

I've tried this with both double quotes before the <link declaration,
but keep getting a parse error.
Help appreciated.
Thanks.
Dave.