When should htmlentities, strip_tags, and htmlspecialchars be used in PHP code to ensure data integrity and security?

To ensure data integrity and security in PHP code, htmlentities, strip_tags, and htmlspecialchars should be used when dealing with user input that will be displayed on a web page. htmlentities should be used to encode special characters into HTML entities, preventing cross-site scripting attacks. strip_tags should be used to remove any HTML or PHP tags from the input to prevent code injection. htmlspecialchars should be used to convert special characters to HTML entities, protecting against XSS attacks. It's important to use these functions when displaying user input to prevent malicious code execution and maintain the integrity of the data being displayed.

// Example code snippet using htmlentities, strip_tags, and htmlspecialchars
$user_input = "<script>alert('XSS attack!')</script>";
$encoded_input = htmlentities(strip_tags($user_input));
echo htmlspecialchars($encoded_input);