Undefined variable problem

Just starting to learn oo php. Probably a simple problem.

Here is some of the code from header.php

$serverName = "server\\exactsqlexpress"; 
$connectionInfoIntranet = array( "Database"=>"intranet", "UID"=>"???", "PWD"=>"???");
$connectionInfoPpp = array( "Database"=>"ppp", "UID"=>???", "PWD"=>"???");

Here is some of the code from dbIntranet.php

include("header.php");

class dbIntranet {

	private $connIntranet;
	
	function __construct(){
			$this->open_connection();
	}
	
	public function open_connection() {
			$this->connIntranet = sqlsrv_connect($serverName, $connectionInfoIntranet); <-----LINE 13
	}

I keep getting an error:

Notice: Undefined variable: serverName in C:\inetpub\wwwroot\portal2\inc\dbIntranet.php on line 13

What's up?