What are some common mistakes to avoid when implementing form validation in PHP, particularly with regards to special characters and input sanitization?

One common mistake to avoid when implementing form validation in PHP is not properly sanitizing user input for special characters. This can lead to security vulnerabilities such as SQL injection or cross-site scripting attacks. To prevent this, always sanitize user input using functions like htmlspecialchars() or mysqli_real_escape_string() before processing or storing it.

// Sanitize user input for special characters
$username = htmlspecialchars($_POST['username']);
$email = filter_var($_POST['email'], FILTER_SANITIZE_EMAIL);

// Use mysqli_real_escape_string for database queries
$mysqli = new mysqli("localhost", "username", "password", "database");
$username = $mysqli->real_escape_string($_POST['username']);
$email = $mysqli->real_escape_string($_POST['email']);