I’m not completely sure what you are trying to accomplish, but I think that you might want to take a look at Zapier.
Zapier will allow you to send leads from one ActiveCampaign account to another quite easily, and allow you to map fields however you would like, including adding your own fields during the process.
Check it out and let me know if that will help you accomplish what you are trying to accomplish, if not I’m happy to keep helping!
You are right then the easiest place to start sending lead data out of AC is with a webhook.
I would start by inspecting the data that AC sends when a webhook is triggered using a free service like http://requestb.in/ or https://ngrok.com/ (requestb.in is way easier to get started).
You can add a webhook from the manage webhooks screen of the settings → developers tab.
Your custom PHP script (clientweb.com/lead.php) will need to be able to accept this http POST request, and process the data (save to a DB) on your end. If you need help getting started with accepting POST requests I would recommend downloading Postman (https://www.getpostman.com/) for practice sending, and reading up on the web.