How do I verify users of my PHP application?

Question

While installing an application onto a client's server, I would like to make sure that the client (or a future developer for them, etc) does not copy my application and place it on other domains/servers/local servers.

How can I verify that my application is running on the server I installed it on? I do not want any substantial lag in the script every time it runs, so I assume a 'handshake' method is not appropriate.

I was thinking the script could request a PHP page on my own server every time it runs. This could send my server their server info and domain name, which my script can check against a database of accepted clients. If the request is invalid, my server handles the work of emailing me the details so I can follow it up. This should not slow down the client's script as it isn't expecting a response, and will still operate on their 'invalid' server until I can investigate this and follow it up with them personally.

If this is the best method (or if there is better), what PHP call should I be making to request my server's script? file_get_contents, curl and similar seem to always retrieve the response, which I don't need.

UPDATE

Thank you all for your responses. I completely understand that PHP is open source and should be freely available to edit. I should have stated more clearly initially, but my intentions were for this verification method to assist me in finding anyone breaching my license agreement. The application is covered under a license, but I would also like to include this check so that I can monitor an initial misuse of my application.

Hence, somebody may still breach my license and it would most likely go unnoticed, but if I implement this script I have the advantage of any 'lazy robbers' who don't break apart my application and remove the verifier before ripping it.

Does this justify the use of such a script? If so, is cURL my best option?

Answer 1

Any checking code for verification is easily replaced with a return true;. Look at the faq at https://stackoverflow.com/tags/php/info :

Q. Can I protect my PHP code from theft? If so, how?
A. There is no effective technical solution to protect, encode or encrypt PHP source code. There are many products that offer some levels of protection, but all can be broken with time and effort. Your best option is not a technical solution, but a legal solution in the form of a license agreement.