What are some best practices for structuring HTML forms and PHP scripts to ensure proper functionality, error handling, and security when processing user input?

One best practice for structuring HTML forms and PHP scripts is to validate user input on both the client-side and server-side to ensure data integrity and prevent malicious input. This can be done by using HTML form validation attributes and PHP validation functions. Additionally, implementing error handling to display meaningful error messages to users can improve user experience and help with troubleshooting. Finally, sanitizing user input before processing it can prevent SQL injection and other security vulnerabilities.

<?php
// Server-side validation
if ($_SERVER["REQUEST_METHOD"] == "POST") {
    $username = $_POST["username"];
    $password = $_POST["password"];
    
    // Validate username and password
    if (empty($username) || empty($password)) {
        $error = "Username and password are required.";
    } else {
        // Sanitize user input
        $username = htmlspecialchars($username);
        $password = htmlspecialchars($password);
        
        // Process user input
        // Add code here to handle user input securely
    }
}
?>