Hey guys,

So I can't seem to figure out why my live server is throwing this error. This works perfectly on my local server. Can anyone help me figure out how to fix this, or lead me in the right direction as to fixing this so my login and registration scripts work?

This is the error that shows:

"Notice: Undefined variable: mysqli in /home/public_html/login.php on line 44
Fatal error: Uncaught Error: Call to a member function prepare() on null in /home/public_html/login.php:44 Stack trace: #0 {main} thrown in /home/public_html/login.php on line 44
"

My code is below.
Login.php

<?php
// Initialize the session
session_start();

// Check if the user is already logged in, if yes then redirect him to welcome page
if(isset($_SESSION["loggedin"]) && isset($_SESSION["loggedin"]) === true){
    header("location: profile.php");
    exit;
}

// Include config file
require_once "config.php";

// Define variables and initialize with empty values
$username = $password = "";
$username_err = $password_err = "";

// Processing form data when form is submitted
if($_SERVER["REQUEST_METHOD"] == "POST"){

    // Check if username is empty
    if(empty(trim($_POST["username"]))){
        $username_err = "Please enter username.";
    } else{
        $username = trim($_POST["username"]);
    }

    // Check if password is empty
    if(empty(trim($_POST["password"]))){
        $password_err = "Please enter your password.";
    } else{
        $password = trim($_POST["password"]);
    }

    // Validate credentials
    if(empty($username_err) && empty($password_err)){
        // Prepare a select statement
        $sql = "SELECT id, username, password FROM users WHERE username = ?";

        if($stmt = $mysqli->prepare($sql)){
            // Bind variables to the prepared statement as parameters
            $stmt->bind_param("s", $param_username);

            // Set parameters
            $param_username = $username;

            // Attempt to execute the prepared statement
            if($stmt->execute()){
                // Store result
                $stmt->store_result();

                // Check if username exists, if yes then verify password
                if($stmt->num_rows == 1){
                    // Bind result variables
                    $stmt->bind_result($id, $username, $hashed_password);
                    if($stmt->fetch()){
                        if(password_verify($password, $hashed_password)){
                            // Password is correct, so start a new session
                            session_start();

                            // Store data in session variables
                            $_SESSION["loggedin"] = true;
                            $_SESSION["id"] = $id;
                            $_SESSION["username"] = $username;

                            // Redirect user to welcome page
                            header("location:profile.php");
                        } else{
                            // Display an error message if password is not valid
                            $password_err = "The password you entered was not valid.";
                        }
                    }
                } else{
                    // Display an error message if username doesn't exist
                    $username_err = "No account found with that username.";
                }
            } else{
                echo "Oops! Something went wrong. Please try again later.";
            }
        }

        // Close statement
        $stmt->close();
    }

    // Close connection
    $mysqli->close();
}
?>

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Login</title>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">
    <style type="text/css">
      .wrapper{
        padding: 20px;
      }
    </style>
</head>
<body class="bg-dark" style="padding: 0; margin: 0;">
  <div class="container mt-4">
    <div class="row">
      <div class="col-lg-4 offset-lg-4 bg-light rounded">
        <div class="wrapper">
            <h2>Login</h2>
            <p>Please fill in your credentials to login.</p>
            <form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>" method="post">
                <div class="form-group <?php echo (!empty($username_err)) ? 'has-error' : ''; ?>">
                    <label>Username</label>
                    <input type="text" name="username" class="form-control" value="<?php echo $username; ?>">
                    <span class="help-block"><?php echo $username_err; ?></span>
                </div>
                <div class="form-group <?php echo (!empty($password_err)) ? 'has-error' : ''; ?>">
                    <label>Password</label>
                    <input type="password" name="password" class="form-control">
                    <span class="help-block"><?php echo $password_err; ?></span>
                </div>
                <div class="form-group">
                    <input type="submit" class="btn btn-primary" value="Login">
                </div>
                <p>Don't have an account? <a href="register.php">Sign up now</a>.</p>
            </form>
            <p class="text-center">Powered by <a href="index.php">SampleCompany</a></p>
        </div>
      </div>
    </div>

  </div>

</body>
</html>

Also my register page won't input the data into the database, which has been tested and is connected correctly.
register.php

<?php
// Include config file
require_once "config.php";

// Define variables and initialize with empty values
$username = $password = $confirm_password = "";
$username_err = $password_err = $confirm_password_err = "";

// Processing form data when form is submitted
if($_SERVER["REQUEST_METHOD"] == "POST"){

    // Validate username
    if(empty(trim($_POST["username"]))){
        $username_err = "Please enter a username.";
    } else{
        // Prepare a select statement
        $sql = "SELECT id FROM users WHERE username = ?";

        if($stmt = $mysqli->prepare($sql)){
            // Bind variables to the prepared statement as parameters
            $stmt->bind_param("s", $param_username);

            // Set parameters
            $param_username = trim($_POST["username"]);

            // Attempt to execute the prepared statement
            if($stmt->execute()){
                // store result
                $stmt->store_result();

                if($stmt->num_rows == 1){
                    $username_err = "This username is already taken.";
                } else{
                    $username = trim($_POST["username"]);
                }
            } else{
                echo "Oops! Something went wrong. Please try again later.";
            }
        }

        // Close statement
        $stmt->close();
    }

    // Validate password
    if(empty(trim($_POST["password"]))){
        $password_err = "Please enter a password.";
    } elseif(strlen(trim($_POST["password"])) < 6){
        $password_err = "Password must have atleast 6 characters.";
    } else{
        $password = trim($_POST["password"]);
    }

    // Validate confirm password
    if(empty(trim($_POST["confirm_password"]))){
        $confirm_password_err = "Please confirm password.";
    } else{
        $confirm_password = trim($_POST["confirm_password"]);
        if(empty($password_err) && ($password != $confirm_password)){
            $confirm_password_err = "Password did not match.";
        }
    }

    // Check input errors before inserting in database
    if(empty($username_err) && empty($password_err) && empty($confirm_password_err)){

        // Prepare an insert statement
        $sql = "INSERT INTO users (username, password) VALUES (?, ?)";

        if($stmt = $mysqli->prepare($sql)){
            // Bind variables to the prepared statement as parameters
            $stmt->bind_param("ss", $param_username, $param_password);

            // Set parameters
            $param_username = $username;
            $param_password = password_hash($password, PASSWORD_DEFAULT); // Creates a password hash

            // Attempt to execute the prepared statement
            if($stmt->execute()){
                // Redirect to login page
                header("location: login.php");
            } else{
                echo "Something went wrong. Please try again later.";
            }
        }

        // Close statement
        $stmt->close();
    }

    // Close connection
    $mysqli->close();
}
?>

<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Sign Up</title>
    <link rel="stylesheet" href="https://stackpath.bootstrapcdn.com/bootstrap/4.3.1/css/bootstrap.min.css" integrity="sha384-ggOyR0iXCbMQv3Xipma34MD+dH/1fQ784/j6cY/iJTQUOhcWr7x9JvoRxT2MZw1T" crossorigin="anonymous">
    <style type="text/css">
      .wrapper{
        padding: 20px;
      }
    </style>
</head>
<body class="bg-dark" style="padding: 0; margin: 0;">
  <div class="container mt-4">
    <div class="row">
      <div class="col-lg-4 offset-lg-4 bg-light rounded">
        <div class="wrapper">
            <h2>Sign Up</h2>
            <p>Please fill this form to create an account.</p>
            <form action="<?php echo htmlspecialchars($_SERVER["PHP_SELF"]); ?>" method="post">
                <div class="form-group <?php echo (!empty($username_err)) ? 'has-error' : ''; ?>">
                    <label>Username</label>
                    <input type="text" name="username" class="form-control" value="<?php echo $username; ?>">
                    <span class="help-block"><?php echo $username_err; ?></span>
                </div>
                <div class="form-group <?php echo (!empty($password_err)) ? 'has-error' : ''; ?>">
                    <label>Password</label>
                    <input type="password" name="password" class="form-control" value="<?php echo $password; ?>">
                    <span class="help-block"><?php echo $password_err; ?></span>
                </div>
                <div class="form-group <?php echo (!empty($confirm_password_err)) ? 'has-error' : ''; ?>">
                    <label>Confirm Password</label>
                    <input type="password" name="confirm_password" class="form-control" value="<?php echo $confirm_password; ?>">
                    <span class="help-block"><?php echo $confirm_password_err; ?></span>
                </div>
                <div class="form-group">
                    <input type="submit" class="btn btn-primary" value="Submit">
                    <input type="reset" class="btn btn-default" value="Reset">
                </div>
                <p>Already have an account? <a href="login.php">Login here</a>.</p>
            </form>
            <p class="text-center">Powered by <a href="index.php">SampleCompany</a></p>
        </div>
      </div>
    </div>
  </div>

</body>
</html>

Thank you for any help