For developers, WHMCS offers a Software Licensing Addon that allows you to sell and protect your own PHP or web-based applications. Pricing Plans - WHMCS
You will not receive security patches, leaving your system vulnerable to hackers. whmcs licensing
You can buy directly from or through an authorized reseller (like cPanel or some hosting providers). Avoid random eBay or Fiverr sellers – licenses bought secondhand are often nullified. For developers, WHMCS offers a Software Licensing Addon