In the context of PHP programming, what are some common mistakes that beginners make when writing code, as evident in the forum thread discussions?

Common mistakes that beginners make in PHP programming include using outdated functions, not properly sanitizing user input, and neglecting error handling. To solve these issues, it's important to stay updated on best practices, always sanitize user input to prevent security vulnerabilities, and implement error handling to catch and handle any unexpected issues that may arise.

// Example code snippet demonstrating proper user input sanitization
$user_input = $_POST['user_input'];
$clean_input = htmlspecialchars($user_input);