MySQLi : Fatal error: Call to a member function query() on a non-object

Question

I know this type of question answered some where but I am unable to find and solve my issue. I Use MySQLi for login, its work properly on localhost but fire error (Fatal error: Call to a member function query() on a non-object /* PATH */ on line 14) on server. My Localhost version is (php : 5.5.12 and apache 2.4.9) and server version is (php 5.3.27).

Here is my php code

    <?php 
    include 'includes/loginheader.php';
    if (isset($_REQUEST['email']))
    {

        $username = $_REQUEST['email'];
        $password = $_REQUEST['password'];
        //$username = mysqli_real_escape_string($mysqli, $username);
        //$query = mysqli_query($con,"SELECT * FROM user");
        //$result = mysqli_query($query);


        $sql = "SELECT * FROM admin";
        $result = $con->query($sql);

        if(mysqli_num_rows($result) == 0) // User not found. So, redirect to login_form again.
        {
            ?>
            <script>
                $( document ).ready(function() {
                    $("#diverrorpanel").removeClass('hide');
                    $("#diverrormsg").text('Username/ Email Not Found');
                });
            </script>
            <?php
        }
        $userData = mysqli_fetch_array($result, MYSQLi_ASSOC);
        $hash = hash('sha256', $userData['salt'] . hash('sha256', $password) );
        if($hash != $userData['password']) // Incorrect password. So, redirect to login_form again.
        {
            ?>
            <script>
                $( document ).ready(function() {
                    $("#diverrorpanel").removeClass('hide');
                    $("#diverrormsg").text('Invalid Password');
                });
            </script>
            <?php
        }else{ // Redirect to home page after successful login.
            ob_start();
            session_start();
            $_SESSION['email'] = $username;

            header('Location: dashboard.php');
            //echo "Success";
        }

    }
?>