First when a user logged in put the logged in user in to a session

$sql = "SELECT * FROM users WHERE (email='$email' OR username='$email') AND password='$password' LIMIT 1";
           $result = mysqli_query($conn, $sql);
           if (mysqli_num_rows($result) > 0) {
				$row = mysqli_fetch_assoc($result);
				// get id of created user
				$reg_user_id = $row['id']; 
                // put logged in user into session array
				$_SESSION['user'] = getUserById($reg_user_id); 
// do your code here
}

the getUserById() function is used to get a session user information

// Get user info from user id
	function getUserById($id)
	{
		global $conn;
		$sql = "SELECT * FROM users WHERE id=$id LIMIT 1";
		$result = mysqli_query($conn, $sql);
		$user = mysqli_fetch_assoc($result);
		// returns user in an array format: 
		return $user; 
	}

then check if a user logged in or not using the IsLoggedIn(0 function as below

function isLoggedIn()
{
	if (isset($_SESSION['user'])) {
		return true;
	}else{
		return false;
	}
}
---------you may also restrict a resource based on a user type---------
function isLoggedIn()
{
  	if (isset($_SESSION['user']['role']) == "Admin" || isset($_SESSION['user']['role']) == "Author"){
		return true;
	}else{
		return false;
	}
}

finally write the following line of code at the beginning of your home page/ admin page based on your need.

if (!isLoggedIn())
 {
       $_SESSION['messages'] = " You must log in first to access this page.";
		header('location: ' . BASE_URL . 'login');
	}

that's it all  done.