How do you get the docroot directory?

// server document directory
$dir = $_SERVER['DOCUMENT_ROOT'];

// wordpress install directory 
$dir = ABSPATH;

to include safely with wordpress

add_action('plugins_loaded', 'load_a_file');
function load_a_file(){
    if( is_admin() ){
        $dir = $_SERVER['DOCUMENT_ROOT'];
        include_once( $dir . '/file.php' );
    }
}