In what situations should PHP developers use htmlentities or htmlspecialchars functions to handle user input and prevent security vulnerabilities in their code?

PHP developers should use htmlentities or htmlspecialchars functions to handle user input whenever they are displaying user-generated content on a webpage to prevent cross-site scripting (XSS) attacks. These functions will encode special characters in the input, making it safe to output on the page without executing any scripts embedded in the input.

// Using htmlentities to encode user input before displaying it on a webpage
$userInput = "<script>alert('XSS attack');</script>";
$encodedInput = htmlentities($userInput);
echo $encodedInput;