Are there any specific security concerns to keep in mind when allowing users to input custom markup language in PHP?

When allowing users to input custom markup language in PHP, a significant security concern is the risk of Cross-Site Scripting (XSS) attacks. To mitigate this risk, it is crucial to properly sanitize and validate user input before rendering it on the webpage. One way to do this is by using functions like htmlspecialchars() to encode special characters and prevent them from being interpreted as HTML or JavaScript code.

// Sanitize user input to prevent XSS attacks
$user_input = "<script>alert('XSS attack!');</script>";
$sanitized_input = htmlspecialchars($user_input, ENT_QUOTES, 'UTF-8');

echo $sanitized_input;