What steps should be taken to ensure the proper handling of user input in PHP code to prevent errors and vulnerabilities?

To ensure the proper handling of user input in PHP code to prevent errors and vulnerabilities, it is essential to sanitize and validate all user input before using it in your application. Sanitizing input involves removing any potentially harmful characters or code, while validation ensures that the input meets the expected format and criteria. By implementing these measures, you can protect your application from common security threats such as SQL injection, cross-site scripting (XSS), and other forms of attacks.

// Sanitize and validate user input
$username = filter_input(INPUT_POST, 'username', FILTER_SANITIZE_STRING);
$email = filter_input(INPUT_POST, 'email', FILTER_VALIDATE_EMAIL);

// Check if input is valid
if ($username && $email) {
    // Process the sanitized and validated input
    // Your code here
} else {
    // Handle invalid input
    echo "Invalid input. Please enter a valid username and email.";
}