What are the potential pitfalls of directly inserting PHP variables into form fields?

Directly inserting PHP variables into form fields can pose a security risk known as cross-site scripting (XSS). This vulnerability allows attackers to inject malicious scripts into the form fields, potentially compromising the security of your website and the data of your users. To prevent this, always sanitize and validate user input before inserting it into form fields.

<form>
  <input type="text" name="username" value="<?php echo htmlspecialchars($username); ?>">
</form>