What could be the issue PHP function error in Visual Studio Code [duplicate]

I'm developing PHP pages for my website registration form but I've seem to run into this error - "Undefined function 'mysql_error'. intelephense(1010)", I've seen things online saying it's my Intelliphense Environment PHP version that doesn't register that function. The version is 8.0.0. Any help?

Here's the PHP page:

    <?php
    require('db.php');
    session_start();
    // When form submitted, check and create user session.
    if (isset($_POST['username'])) {
        $username = stripslashes($_REQUEST['username']);    // removes backslashes
        $username = mysqli_real_escape_string($con, $username);
        $password = stripslashes($_REQUEST['password']);
        $password = mysqli_real_escape_string($con, $password);
        // Check user is exist in the database
        $query    = "SELECT * FROM `users` WHERE username='$username'
                     AND password='" . md5($password) . "'";
        $result = mysqli_query($con, $query) or die(mysql_error());
        $rows = mysqli_num_rows($result);
        if ($rows == 1) {
            $_SESSION['username'] = $username;
            // Redirect to user dashboard page
            header("Location: dashboard.php");
        } else {
            echo "<div class='form'>
                  <h3>Incorrect Username/password.</h3><br/>
                  <p class='link'>Click here to <a href='login.php'>Login</a> again.</p>
                  </div>";
        }
    } else {
?>
<?php
    }
?>