What are common security risks associated with using user input directly in PHP scripts?

Common security risks associated with using user input directly in PHP scripts include SQL injection, cross-site scripting (XSS), and remote code execution. To mitigate these risks, it is important to sanitize and validate user input before using it in your PHP scripts. Example PHP code snippet to sanitize user input using the `filter_input` function:

$user_input = filter_input(INPUT_POST, 'user_input', FILTER_SANITIZE_STRING);