What potential security risks are associated with not validating user input in PHP, as seen in the provided code snippet?

Not validating user input in PHP can lead to security vulnerabilities such as SQL injection, cross-site scripting (XSS), and other forms of attacks. To mitigate these risks, it is essential to sanitize and validate user input before using it in any database queries or outputting it to the browser.

// Original code snippet without input validation
$username = $_POST['username'];
$password = $_POST['password'];

// Fixed code snippet with input validation
$username = isset($_POST['username']) ? htmlspecialchars($_POST['username']) : '';
$password = isset($_POST['password']) ? htmlspecialchars($_POST['password']) : '';