What are some potential pitfalls when manipulating variables in PHP?

One potential pitfall when manipulating variables in PHP is not properly sanitizing user input, which can lead to security vulnerabilities such as SQL injection or cross-site scripting attacks. To avoid this, always validate and sanitize user input before using it in your code.

// Example of sanitizing user input using the filter_var function
$user_input = $_POST['user_input'];
$clean_input = filter_var($user_input, FILTER_SANITIZE_STRING);