Can you please elaborate your application/server setup?
But from what you say so far, I would assume you expose an interface to Selenium on your server via PHP or something for users to use. In which case, what they run will run Selenium on "your" server, not their local browser. The former is what happens often and easy to set up, whether it works well is another matter. The latter is very hard to implement due to cross machine security and interaction, especially over the internet.