PHP $_SESSION Empty on Next Page

PHP $_SESSION is empty during AJAX calls. First thing to check is register_globals must be Off. PHP $_SESSION return empty string when user navigates to the next page. When session ID is not invoked properly, the reason can be that the session.save_path is not set.
$_SESSION['username'] = $_POST['username']; The session variables are not set properly.

Now in this condition the session variables return blank on the next page.
echo $_SESSION['username']; returns blank string.

Checklist for Corrective Measures:

•  Add the following lines at the start of your PHP file:

@ini_set('session.save_path','/home/<UserID>/tmp/sessions');
session_set_cookie_params(3200, '/', $_SERVER['HTTP_HOST']);
session_start();

•  Make sure that the same sesison ID is in use. The session ID is needed to be saved in Javascript or database. 

session_id($_POST['session_id']);

Use session_id() to get the current session. This session ID can be sent to the client in form of values in Javascript variable.

• Normally the session_start() must be called before request headers are being sent, in order to set or retrieve any session variable value using the keyword $_SESSION.
  The session variables has been set or not can re-verified using the following line of codes:

 isset($_SESSION['username']);

• Check the satus of register_globals.

 Paste the code phpinfo(); in the PHP script on server. If the value is register_globals = On, then it is needed to be changed using a php.ini file.
Create a php.ini file in the root folder and add the following line of code.

register_globals = Off

• Create a php.ini file in the root folder and add the following line of codes:

session.save_path = "/home/<UserID>/tmp/sessions"
register_globals = Off

Also add the following lines in .htaccess file in the same folder:

<IfModule mod_suphp.c>
  suPHP_ConfigPath /home/<UserID>
  <Files php.ini>
    order allow,deny
    deny from all
  </Files>
</IfModule>

The <Files> tag makes sure that anyone form outside can't access the php.ini in the form of a web page URL.

Related Links:
PHP .htaccess