At first, I consider $_REQUEST not very useful. I have not use it for years.
Secondly, when the $_REQUEST contains data that application-specific, HTTP_ACCEPT is browser specific and should not really affect on how application works. The same for other HTTP request variables that are in $_SERVER.
$_REQUEST contains GET + POST + COOKIE which are the request.
$_SERVER contains environment settings HTTP headers are part of the environment, request URI is processed by the web server therefor its in $_SERVER.
Logic without the fatal effects.
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