How to resolve invalid use of $_SERVER in PHP?

If using $_SERVER in PHP is not working, there could be several reasons and solutions:

1. Make sure that the web server is already running before using $_SERVER. If the web server is not started, the $_SERVER variable will not work properly. You can try starting or restarting the web server.
2. Check the configuration file of the web server to ensure that the $_SERVER variable has not been disabled or altered. Sometimes, the configuration file of the web server may restrict or modify the $_SERVER variable. You can examine the configuration file and adjust the relevant settings.
3. Check to ensure that the usage of the $_SERVER variable is correct. $_SERVER is a superglobal variable that contains information about the server and execution environment, and you can use print_r($_SERVER) to view its contents. Make sure to use the correct key names and syntax when using $_SERVER.
4. If a proxy server or load balancer is being used, it may be necessary to pass the $_SERVER variable to the application. This can be achieved by configuring the proxy server or load balancer. For more information, refer to the relevant documentation or consult with the server administrator.
5. If a caching mechanism is utilized, it may be necessary to refresh the cache to ensure that the $_SERVER variable is updated. Sometimes, the caching mechanism may store an outdated value for the $_SERVER variable, rendering it invalid. Resolving this issue could involve attempting to clear or disable the cache.

If none of the above methods work, it may be necessary to further examine the server environment and code logic, or consider discussing the issue with other developers or server administrators.