How should variables be handled and passed between HTML, PHP, and MySQL to ensure proper functionality?

Variables should be properly sanitized and validated before being passed between HTML, PHP, and MySQL to prevent security vulnerabilities and ensure data integrity. To achieve this, use PHP functions like mysqli_real_escape_string() to sanitize user inputs before storing them in the database, and htmlspecialchars() to prevent XSS attacks when displaying data on the HTML page. Additionally, use prepared statements to prevent SQL injection attacks when querying the database.

// Example of sanitizing user input before storing it in the database
$input = mysqli_real_escape_string($connection, $_POST['input']);

// Example of displaying data on HTML page with htmlspecialchars
echo htmlspecialchars($data['name']);

// Example of using prepared statements for querying the database
$stmt = $connection->prepare("SELECT * FROM table WHERE id = ?");
$stmt->bind_param("i", $id);
$stmt->execute();
$result = $stmt->get_result();