Can HTML be processed server-side as well as client-side, and how does this relate to PHP security measures?

HTML can be processed both server-side and client-side. When processing HTML server-side using PHP, it is important to properly sanitize user input to prevent cross-site scripting attacks. This can be done by using PHP functions like htmlspecialchars() to encode special characters in user input before displaying it on a webpage.

// Sanitize user input before displaying it on a webpage
$userInput = "<script>alert('XSS attack!');</script>";
$sanitizedInput = htmlspecialchars($userInput);
echo $sanitizedInput;