This description will show you, how to use the Workflow Designer for Webforms.
If you already use the internal VtigerCRM Webforms, you maybe know the common problems.
1. Performance
2. You must share your CRM URL
3. Limited opportunities to prevent duplicates

The Workflow Designer have a build in HTTP Handler module, which could be access remotely by HTTP(s). But it must not be called directly by clients, because you must enable single IP’s/IP Ranges, which are allowed to access a URL and trigger a Workflow. That’s why you need a “Gateway” Script.

Only this Script is called by clients and internally forward the call to your VtigerCRM Workflow Designer. This open some opportunities you not have with internal Webforms, because you could use proxies, HTTP Auth or Client Certification to access the HTTP Handler.

You found an example of such file here:
This simple version don’t support any security features and you need a PHP developer to implement this. (Because it is very individual)