What are the potential security risks of allowing HTML tags in user input fields in PHP?

Allowing HTML tags in user input fields can lead to security risks such as Cross-Site Scripting (XSS) attacks, where malicious scripts can be injected into the webpage and executed in the context of the user's browser. To prevent this, you can use the `htmlspecialchars()` function in PHP to escape special characters and prevent them from being interpreted as HTML code.

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