获取相对于Apache的文档根目录的文件路径

Given $abs_path='/var/www/foo/html/bar/bla/bla';, how do I get the path relative to Apache's document root?

I recognize I could just use somethinglike $rel_path=str_replace ($_SERVER['DOCUMENT_ROOT'],'',$abs_path,1 );, but curious if there is a built in function.