In what scenarios would using htmlentities in PHP code be necessary or recommended?

Using htmlentities in PHP code is necessary or recommended when you want to output user-generated content on a webpage to prevent cross-site scripting (XSS) attacks. By using htmlentities, special characters in the content will be converted to their HTML entities, making it safe to display on a webpage without executing any malicious scripts embedded in the content.

$user_input = "<script>alert('XSS attack!');</script>";
echo htmlentities($user_input);