Checkpassword Issue with PHPass

问题:

I am using phpass for password encryption. When I am adding a user the password is getting hashed and being stored in the database. But when I try logging with the user credentials stored in the database checkpassword returns an empty string. Below given is the code I am using for checking the passwords.

    function PackageAccess($username,$password)
    {
        $db = new mysqli(DB_SERVER,DB_USER,DB_PASSWORD,DB_NAME) or die("Could not Connect");
        $hasher = new PasswordHash(8,FALSE);
        $accessvalue = 0;
        $query = "select passwrd from usermaster where username=?";
        if ($stmt=$db->prepare($query))
        {
            $stmt->bind_param("s",$username);
            $stmt->execute();
            $stmt->bind_result($a);
            while($stmt->fetch())
            {
                if($hasher->CheckPassword($password,$a))
                {
                    $accessvalue = 1;
                }
                else
                    {
                        $accessvalue = 0;
                    }
            }
            $stmt->close();
        }
        $db->close();
        $hasher = null;
        return $accessvalue;
    }

This code always gives me an empty string and neither true nor false. Hoping somebody can throw some light on this.