旧的自定义php代码管理面板登录问题

我有一个网站的管理员登录问题。该网站是7-8年前由一位开发人员从头开始自定义编码的。它运行在php5.3版本上。我最近把网站搬到了一个配置更好的新服务器上。自从网站被移到此服务器后，我就无法登录到网站的自定义编码管理控件。

我不知道我到底错过了什么

data.php代码

<?
session_start();
$dbh = @mysql_connect('localhost','mangal_nwwwman','Mangu@123');
mysql_select_db("mangal_qmangi", $dbh);
if (!$dbh) {
  echo "ERROR: There was an error connecting to the database server.  Please try again later.";
  exit;
}
/*if (!@mysql_select_db("mangal_qmangi", $dbh)) {
  echo "ERROR: There was an error connecting to the database.  Please try again later.";
  exit;
}*/
function dateconverter ($data) {
    $date = substr($data,5,2)."-".substr($data,8,2)."-".substr($data,0,4);
    return $date;
}
include("security.php");
?>

security.php代码

<?
if(!isset($userid)){
if((!isset($PHP_AUTH_USER)) && (!isset($PHP_AUTH_PW))) {
    Header("WWW-Authenticate: Basic realm='"Mangalorean.com Admin Security System'"");
    Header("HTTP/1.0 401 Unauthorized");
    echo  "Authentication Problem...'n";
    exit;
    }else{
    $query = "select * from managers where username='$PHP_AUTH_USER' and password='$PHP_AUTH_PW'";
    $manager_query = mysql_query($query);
    echo mysql_error();
    $manager_row = mysql_fetch_array($manager_query);
    if ($manager_row>0)
        {
        $username=$PHP_AUTH_USER;
        $userid=$manager_row[userid];
        $permission=$manager_row[permission];
        $name=$manager_row[name];
        session_start();
        session_register ("userid");
        session_register ("permission");
        session_register ("name");
        header("location:index.php");
        exit;
    }
    }
    Header( "WWW-authenticate:  Basic  realm='"Mangalorean.com Admin Security System'"");
    Header( "HTTP/1.0  401  Unauthorized");
    echo  "<b>Authentication Problem...</b><br><br>Contact the administrator.";
    exit;
}
?>

有人能帮我了解发生了什么吗？为什么这不起作用？