title: Make button enabled for only one user on webpage
body: <p>I'm developing a website that is suppoused to control some devices that rotate when pushing one button. However, I only want one user at a time to be able to push the button.</p>
  
  <p>That is, when a user enters the page, check if it is the "first" or only user that is there. In case he is, he can use the button freely. In case he isn't, the button appears as disabled.</p>
  
  <p>What would be the best way to implement this? I'm using a classic design of html+js+php.</p>
  
  <p>Thank you.</p>