Hello Friends

I just changed my host, I am getting this error on my scrip which worked fine with my old host.
Please can anyone help me with this?
this is the error message I get

Warning: Unknown: Your script possibly relies on a session side-effect which existed until PHP 4.2.3. Please be advised that the session extension does not consider global variables as a source of data, unless register_globals is enabled. You can disable this functionality and this warning by setting session.bug_compat_42 or session.bug_compat_warn to off, respectively in Unknown on line 0

This is my script
PHP Code:
if (!$_SESSION['username']) 
{header("location: login.php?login=error");

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<title>Treasure Care Home Health :: Administrator's Page</title>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-1" />

<style type="text/css">
@import url("_mmServerScripts/.subheading.css");
<body bgcolor="#FFFFFF">
<style TYPE="text/css">
<!--  A:link {text-decoration: none; color:#000055}  A:visited {text-decoration: none; color:#000055}  A:active {text-decoration: none; color:#000022}   A:hover {text-decoration: ; color:#00CC00 }-->
<form action="search.php" method="post">
  <table width="845" border="0" align="center" cellpadding="0" cellspacing="2">
      <td align="left">
        <img border="0" src="logo.png" width="430" height="81"></td>
    <?php // $username = $_GET['username']; //= $_GET['username'];
//$password = $_GET['password'];//= $_GET['password'];
      <td align="center">Recruitment Profile Administrator: You are logged in as<font color="#000066"> <strong><?php echo $_GET['username']; ?></strong></font></td>