What are the potential pitfalls of displaying user data without proper validation and sanitization in PHP?

Displaying user data without proper validation and sanitization in PHP can lead to security vulnerabilities such as SQL injection, cross-site scripting (XSS), and other forms of attacks. To mitigate these risks, always validate and sanitize user input before displaying it on a webpage.

// Example of validating and sanitizing user input before displaying it
$userInput = $_POST['user_input'];

// Validate user input
if (filter_var($userInput, FILTER_VALIDATE_EMAIL)) {
    // Sanitize user input
    $sanitizedInput = htmlspecialchars($userInput);

    // Display sanitized input
    echo $sanitizedInput;
} else {
    echo "Invalid input";
}