PDO connection problem

I am getting very strange type of PDO connection problem. I am using following code to create a pdo object

try {
    $dbase = new PDO("mysql:host=$hostname;dbname=$db", $user, $pass);
  //  echo 'Connected to database';
    }
catch(PDOException $e)
    {
    echo $e->getMessage();
    }

PHP:

Code is in a include file so all pages access Mysql using this code.

I am getting problem only in one page. At first page load everything is fine. this page submit a form to iteself. After submitting form I get this error
SQLSTATE[28000] [1045] Access denied for user 'user_name'@'localhost' (using password: YES) (All other site pages are working fine. There is no issue in creating pdo object and accessing Mysql).

Can somebody help me fixing this issue.