Show alert message automatically after session timeout without manually refresh the page

Question

I have a question on how to automatically show alert message. I have set the time limit to 10s but I need to manually refresh the page then alert message will pop up. Alert message that will be display will tell the user the session is over and reload the page. Here is my code

<?php
        //start session
        session_start();

        //database connection
        $conn = mysqli_connect("localhost","root","","test"); 

        //default timezone
        date_default_timezone_set('Asia/Kuala_Lumpur');

        //if user click login button
        if(!empty($_POST["login"])) 
        {
            //query table to verify inserted value
            $result = mysqli_query($conn,"SELECT * FROM users WHERE username = '" . $_POST["user_name"] . "' and password = '". $_POST["password"]."'");

            //fetch result result row as an associative, a numeric array, or both
            $row  = mysqli_fetch_array($result);

            //if it is true
            if($row) 
            {
                //declare a session for selected value using id and time logged in
                $_SESSION["user_id"] = $row['id']; 
                $_SESSION['timestamp'] = time();            
            } 
            else 
            {
                //redirect to homepage
                echo '<script type="text/javascript">alert("Invalid Username or Password!");window.location = "userlogin_session.php";</script>';
            }
        }

        //check for session timeout
        if(isset($_SESSION['timestamp']))
        {
            //set time limit in seconds
            $expireAfterSeconds = 10;

            //calculate many seconds have passed since the user was last active 
            $secondsInactive = time() - $_SESSION['timestamp'];

            //convert seconds into minutes
            $expireAfter = $expireAfterSeconds / 60 ;

            //check to see if time is equals or above given time limit
            if($secondsInactive >= $expireAfter)
            {
                //kill session.
                session_unset();
                session_destroy();

                //redirect to homepage
                echo '<script type="text/javascript">alert("Session Over");window.location = "userlogin_session.php";</script>';
            }
        }

        //if user click logout button
        if(!empty($_POST["logout"])) 
        {
            //kill session.
            session_unset();        
            session_destroy();
        }
    ?>

Answer 1

You'll need to do it in Javascript, not PHP. You can however send the PHP var to the javascript, or just hard code it (seconds * 1000) and then get it to alert or modal window:

setTimeout(function(){
   alert ('Session timeout message or code here');
}, <?= $timeout; ?>);

Answer 2

This is my full code.

<?php
        //start session
        session_start();

        //database connection
        $conn = mysqli_connect("localhost","root","","test"); 

        //default timezone
        date_default_timezone_set('Asia/Kuala_Lumpur');

        //if user click login button
        if(!empty($_POST["login"])) 
        {
            //query table to verify inserted value
            $result = mysqli_query($conn,"SELECT * FROM users WHERE username = '" . $_POST["user_name"] . "' and password = '". $_POST["password"]."'");

            //fetch result result row as an associative, a numeric array, or both
            $row  = mysqli_fetch_array($result);

            //if it is true
            if($row) 
            {
                //declare a session for selected value using id and time logged in
                $_SESSION["user_id"] = $row['id']; 
                $_SESSION['timestamp'] = time();            
            } 
            else 
            {
                //redirect to homepage
                echo '<script type="text/javascript">alert("Invalid Username or Password!");window.location = "userlogin_session.php";</script>';
            }
        }

        //check for session timeout
        if(isset($_SESSION['timestamp']))
        {
            //set time limit
            $expireAfterSeconds= 10;

            //calculate many seconds have passed since the user was last active 
            $secondsInactive = time() - $_SESSION['timestamp'];
            echo $secondsInactive;

            //check to see if time is equals or above given time limit
            if($secondsInactive >= $expireAfterSeconds)
            {
                //kill session.
                session_unset();
                session_destroy();

                //redirect to homepage
                //echo '<script type="text/javascript">alert("Session Over");window.location = "userlogin_session.php";</script>';
            ?>
                <script>
                    alert("Session Over");
                    window.location = "userlogin_session.php";
                </script>';
            <?php
            }
        }

        //if user click logout button
        if(!empty($_POST["logout"])) 
        {
            //kill session.
            session_unset();        
            session_destroy();
        }
    ?>
    <html>
        <head>
            <title>User Login</title>
        </head>
        <body>
            <?php 
                //if session not exist
                if(empty($_SESSION["user_id"])) 
                { 
                ?>
                    <form action="" method="post" id="frmLogin">
                        <div><?php if(isset($message)) { echo $message; } ?></div>  
                        <div>
                            <div><label for="login">Username</label></div>
                            <div><input name="user_name" type="text"></div>
                        </div>
                        <div>
                            <div><label for="password">Password</label></div>
                            <div><input name="password" type="password"> </div>
                        </div>
                        <div>
                            <div><input type="submit" name="login" value="Login"></span></div>
                        </div>       
                    </form>
                <?php 
                }

                //if session exist
                else 
                { 
                    $result = mysqli_query($conn,"SELECT * FROM users WHERE id = '" . $_SESSION["user_id"] . "'");
                    $row  = mysqli_fetch_array($result);
                ?>  
                    <form action="" method="post" id="frmLogout">
                        <div>
                            Welcome <?php echo ucwords($row['username']); ?>, You have successfully logged in!<br>Click to <input type="submit" name="logout" value="Logout">
                        </div>
                    </form>
                    </div>
                    </div>
                <?php 
                } 
            ?>
        </body>
    </html>