What are the potential security risks associated with using user input directly in PHP scripts, as seen in the code provided?

Using user input directly in PHP scripts can lead to security vulnerabilities such as SQL injection, cross-site scripting (XSS), and command injection. To mitigate these risks, it is crucial to sanitize and validate user input before using it in your code. This can be done by using functions like `htmlspecialchars()` to prevent XSS attacks, `mysqli_real_escape_string()` to prevent SQL injection, and `escapeshellarg()` to prevent command injection.

// Example of sanitizing user input before using it in a PHP script
$user_input = $_POST['user_input']; // Assuming user input is coming from a form POST request

// Sanitize user input to prevent XSS attacks
$sanitized_input = htmlspecialchars($user_input);

// Use the sanitized input in your code
echo "User input: " . $sanitized_input;