MM_Username

I'm working to develop an app & my login function no longer works.  The MM_Username is not being passed between pages.  I've looked at other posts here on this forum, but nothing seems to work.  I have a remote server through GoDaddy; my php.ini file has the below:

register_globals = off
allow_url_fopen = off

expose_php = Off
max_input_time = 60
variables_order = "EGPCS"
extension_dir = ./
upload_tmp_dir = /tmp
precision = 12
SMTP = relay-hosting.secureserver.net
url_rewriter.tags = "a=href,area=href,frame=src,input=src,form=,fieldset="

[Zend]
zend_extension=/usr/local/zo/ZendExtensionManager.so
zend_extension=/usr/local/zo/4_3/ZendOptimizer.so

Here's the code on my login page:

if (!isset($_SESSION)) {
  session_start();
}

$loginFormAction = $_SERVER['PHP_SELF'];
if (isset($_GET['accesscheck'])) {
  $_SESSION['PrevUrl'] = $_GET['accesscheck'];
}

if (isset($_POST['username'])) {
  $loginUsername=$_POST['username'];
  $password=$_POST['password'];
  $MM_fldUserAuthorization = "usr_pfl_perms";
  $MM_redirectLoginSuccess = "log_in_thank_you.php";
  $MM_redirectLoginFailed = "log_in_unsuccesful.php";
  $MM_redirecttoReferrer = false;
  mysql_select_db($database_mySQL, $mySQL);

I know my software is writing the session start();.

Any ideas?

Many thanks.