What is the purpose of using str_replace and htmlspecialchars in the given PHP code?

The purpose of using `str_replace` and `htmlspecialchars` in PHP code is to prevent Cross-Site Scripting (XSS) attacks by sanitizing user input and escaping special characters that could be used for malicious purposes. `str_replace` is used to replace certain characters with their HTML entity equivalents, while `htmlspecialchars` is used to convert special characters to HTML entities to prevent them from being interpreted as code.

// Original code vulnerable to XSS attacks
$user_input = $_POST['user_input'];
echo $user_input;

// Fixed code using str_replace and htmlspecialchars
$user_input = $_POST['user_input'];
$sanitized_input = str_replace("<", "<", $user_input);
$sanitized_input = str_replace(">", ">", $sanitized_input);
$sanitized_input = htmlspecialchars($sanitized_input);
echo $sanitized_input;