What functions or methods in PHP can be used to create a secure and unique activation code for user accounts?

To create a secure and unique activation code for user accounts in PHP, you can use the `uniqid()` function to generate a unique identifier based on the current timestamp. You can then hash this identifier using the `md5()` function to create a more secure activation code. Finally, you can store this activation code in your database along with the user's account information for verification purposes.

$activationCode = md5(uniqid(rand(), true));