title: How can I secure my plugin so only paying users can use it?
body: <p>I'm developing some (wordpress) plugins and I'm planning to have a license fee for whoever want's to use it.</p>
  
  <p>Therefor I need a way to make sure that this plugin is not uploaded to a server where anyone can download it and use it for free.</p>
  
  <p>So I was thinking of using an API key. Valid API key = user can use the plugin. Invalid = plugin does not work.</p>
  
  <p>I've looked at this post <a href="https://stackoverflow.com/questions/1448455/php-api-key-generator">PHP API Key Generator</a> but I don't get much wiser of that.</p>
  
  <p>I also know that since it's PHP, anyone can go into the code and disable API check (I'm just guessing)</p>
  
  <p>What is the best way to secure my plugin? API key? Other ways? Does anyone have link to any good tutorials on the subject?</p>