What steps can be taken to prevent users from entering HTML or PHP code into forms?

To prevent users from entering HTML or PHP code into forms, you can use the htmlspecialchars() function in PHP to convert special characters to their HTML entities. This will ensure that any code entered by the user is displayed as plain text rather than being executed.

$user_input = $_POST['user_input'];
$clean_input = htmlspecialchars($user_input);