Why is it important to validate and sanitize input data in PHP applications?

It is important to validate and sanitize input data in PHP applications to prevent security vulnerabilities such as SQL injection, cross-site scripting (XSS), and other forms of attacks. Validating input ensures that the data meets the expected format and type, while sanitizing input removes any potentially harmful characters or code.

// Validate and sanitize input data
$username = isset($_POST['username']) ? htmlspecialchars($_POST['username']) : '';
$email = isset($_POST['email']) ? filter_var($_POST['email'], FILTER_SANITIZE_EMAIL) : '';