I sometimes see one or the other in my server logs.
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The HTTP headers beginning with
Xare officious headers. They are not specified in the HTTP specification, but just coined by some 3rd party as kind of vendor-specific or “de facto” standard and taken over by many others, like HTTPX-Forwarded-Forin case of proxy servers.The particular HTTP
X-User-Agentheader is often inserted by some independent messenger between the actual client and the server, like a proxy, loadbalancer, transcoder, etc, but also some XMLHttpRequest based libraries. It often identifies the messenger itself. TheUser-Agentheader should still represent the actual client. You also see this header sometimes in emails, this then represents the email program/software responsible for sending the mail.That it is called
HTTP_X_USER_AGENT(uppercased,HTTPprefix, underscores instead of hyphens) is PHP specific.