What are the advantages and disadvantages of using a whitelist for allowed HTML markup in PHP applications?

When allowing user input in PHP applications, it is important to sanitize and validate the input to prevent cross-site scripting (XSS) attacks. One way to do this is by using a whitelist approach for allowed HTML markup. By specifying which HTML tags and attributes are allowed, you can restrict the input to only safe elements. However, using a whitelist can be time-consuming to maintain as new HTML elements and attributes are introduced. It can also be restrictive, potentially limiting the functionality of the application if certain elements are not allowed. Additionally, it may not fully protect against all forms of XSS attacks, as attackers can still find ways to bypass the whitelist.

// Define a whitelist of allowed HTML tags and attributes
$allowed_tags = '<a><p><strong><em><ul><ol><li>';
$allowed_attributes = 'href,title,src';

// Sanitize user input using the whitelist
$clean_input = strip_tags($user_input, $allowed_tags);
$clean_input = htmlspecialchars($clean_input);

// Output the sanitized input
echo $clean_input;