What are the potential risks of not properly validating user inputs in PHP, especially from $_GET and $_POST arrays?

Not properly validating user inputs in PHP, especially from $_GET and $_POST arrays, can lead to security vulnerabilities such as SQL injection, XSS attacks, and data manipulation. To mitigate these risks, it is crucial to validate and sanitize user inputs before using them in your application.

// Example of validating and sanitizing user input from $_POST array
$username = isset($_POST['username']) ? htmlspecialchars($_POST['username']) : '';
$password = isset($_POST['password']) ? htmlspecialchars($_POST['password']) : '';