Hi,
my name is Lluc and I'm a computer engineering student in the Autonomous University of Barcelona.
First of all I want to tell you what you should be looking for. You don't need only an expert in PHP. This project could be divided in 3 basic parts: the website (which will need php, javascript, steam api,...), the database which will basically need mysql and the bot (made in node.js, c#, c++,...)
The bot will be the base of this project since it will be the part that takes the objects and then hands it to the winner. So first, your objective should be having a bot.
The second part should be the database. A good design is the thing that will make the project work. You should have some tables which will be the connection between the bot and the website (like the items in the jackpot, the winner, banned users,...).
And finally, the website. The website can be made in a thousand different ways, and this is something we should take care when we have everything else done. I would recommend javascript to make it look cool and appealing.
The most difficult part of the project would be the Website.
I would have the bot and database done in a week, the rest of the days would be taken by the website.
If you have any questions regarding how I'd do the project please ask.