What are the potential security risks associated with allowing users to post HTML code on a website, and how can PHP be used to prevent these risks?

Allowing users to post HTML code on a website can pose security risks such as cross-site scripting (XSS) attacks, where malicious scripts are injected into the website to steal sensitive information or perform unauthorized actions. PHP can help prevent these risks by sanitizing user input and escaping special characters before displaying the content on the website.

// Sanitize user input to prevent XSS attacks
$user_input = htmlspecialchars($_POST['user_input']);

// Display sanitized user input on the website
echo $user_input;