What are common mistakes made by PHP beginners when creating forms?

One common mistake made by PHP beginners when creating forms is not properly sanitizing user input, which can lead to security vulnerabilities like SQL injection or cross-site scripting attacks. To solve this issue, always sanitize user input using functions like htmlspecialchars() or mysqli_real_escape_string() before using it in SQL queries or displaying it on the webpage.

// Sanitize user input using htmlspecialchars() before displaying it on the webpage
$user_input = htmlspecialchars($_POST['user_input']);

// Sanitize user input using mysqli_real_escape_string() before using it in SQL queries
$user_input = mysqli_real_escape_string($conn, $_POST['user_input']);