instant account creation script

It should be a moderately trivial task for a seasoned PHP developer to resolve such an issue, and so this is one option.

If you're serious about the services you offer it doesn't make sense to use a free script as such a script is less likely to be maintained/updated and won't, generally, come with any form of support. You might be risking a lot on the assumption that it will always work, which is a bad assumption to make.

This is all, however, rather moot, if the free script is maintained/updated/supported in the same way a 'paid for' script would be, or if you are merely using such a free script to understand how such methods work as part of your own development of a similar script.