What honestly you plan in your website? you wanted to make your own web wallet and offers to other users?
I think you can make your own web wallet with JSON-RPC i was honestly hardly to do these when making website in php. i have less knowledge in php i just uses the other command that recommends to me before.. .
Here is the wiki that i use before
https://en.bitcoin.it/wiki/API_reference_(JSON-RPC)
And i think it can help you about your plan..
And here is for PHP developer commands
https://en.bitcoin.it/wiki/PHP_developer_introbase on command listed there i don't see any that can generate valid bitcoin address for 12 word seed..
For more api call i just added this
https://en.bitcoin.it/wiki/Original_Bitcoin_client/API_calls_listEverything in online when making website is try and error.. so you can experiment and which command could gives you what you need..