What potential security risks are associated with allowing PHP to manipulate browser behavior?

Allowing PHP to manipulate browser behavior can lead to security risks such as cross-site scripting (XSS) attacks, where malicious scripts can be injected into web pages and executed in the context of the user's browser. To mitigate this risk, it is important to properly sanitize and validate any user input before using it to manipulate browser behavior.

// Sanitize and validate user input before using it
$user_input = $_POST['user_input'];
$sanitized_input = htmlspecialchars($user_input, ENT_QUOTES, 'UTF-8');

// Use the sanitized input to manipulate browser behavior
echo "<script>alert('$sanitized_input');</script>";