This library lets you use Twilio to validate phone numbers that users enter into your app, for both iPhone and Android.
Your app first posts to validate_phone_number_1.php, which sets everything up. It creates the webpage that your user clicks a link to, and it sends the SMS. After your user opens the app by clicking on the validation link, post to validate_phone_number_2.php.
generate_5_digits.php shows how to generate random-looking 5-digit strings consisting of upper and lowercase letters, and numbers. Very lightweight, doesn't repeat for the first trillion iterations (or so). You can easily use the same method to get random-looking strings of any length.
base_helpers.php contains convert_to_base_array, a useful function for representing positive integers with respect to an arbitrary base.