Would my environment require a paid SQL Server license? -

I have written a plugin for a game that connects to a PHP page and sends information from the game. The PHP page takes this information, connects to a SQL Server 2005 instance, then incorporates the information given in the various tables.

This setup is 'OK' for hosting the database server I am using Apache (WampServer) to host PHP Pages and SQL Server 2005 Express, but as far as development happens, I want to release this plugin to other players. Their version of the plugin will be identical to me because it connects to the remote PHP page to add data to the database in the database or to add query information from the database.

Since the 'clients' are not directly databases, do SQL Server 2005 require some kind of license to pay 'legal' to me in such a way? 'Production Server?' What is the standard definition of?

Note: I am not making any money from this plugin or PHP page, however the subscription payment written to this plugin is required. I'm not affiliated with any developer or game (though I'm in paid membership).