Ensure web app access from a single computer per user
<p>I have developed a web application in PHP for a client. The client is now renting out access to the system to another company on a per user basis.</p>

<p>Is there a way to prevent the secondary company to use a single login and give it to 20 people to use at the same time? I know one can get the IP address of the client machine that is being logged in from, but this is obviously not very reliable method. The answer probably lies in a combination of cookies and tracking things in a database, but my brain gets a bit stuck thinking on how to implement a strategy here.</p>
