In what scenarios would it be advisable to use htmlspecialchars() or htmlentities() in PHP?

It is advisable to use htmlspecialchars() or htmlentities() in PHP when displaying user input on a webpage to prevent cross-site scripting (XSS) attacks. These functions encode special characters in the input, such as <, >, ", ', and &, to prevent them from being interpreted as HTML or JavaScript code. This helps to ensure that the user input is displayed as plain text and not executed as code.

$user_input = "<script>alert('XSS attack!');</script>";
echo "User input: " . htmlspecialchars($user_input);