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The security concern with using the PHP $SERVER variable in a licensing system is that it can be manipulated by attackers. The $SERVER variable contains information about the web server environment, including user IP address, browser type, and version, and the web server software being used.

An attacker can manipulate the $_SERVER variables to simulate requests from different IP addresses or browsers, which can result in a bypass of licensing checks. This can allow unlicensed users to access software or services without payment.

Therefore, it is important to verify the information in the $_SERVER variable and implement additional security measures, such as encryption and server-side validation, to prevent the manipulation of data.