For Clay Creative (the in-house design team for Global Brands) I created two competitions and a one page site in WordPress. The first competition was the most complicated using PHP to check physical bottle cap codes against a hashed version in a database and inform the user whether than had won, lost or been locked out (in the case of multiple entries) using Vanilla JS and cookies.
The second competition was simpler using AJAX to register entrants via WordPress. I wrote this competition (and a simple form validator) as a plugin so it could simply be switched on and then off when the competition ended.
The competitions were all time-limited (6 months being the longest) which is why there are no links. You can visit Clay Creative though.