"Max-Forwards" header of current request.
Upper limit of proxies and gateways that may forward the request. Each proxy and gateway must check this value and decrement the value by one if the request is forwarded, unless the received value is zero (0), the request must not be forwarded but instead must be responded to as the final recipient.
Link: RFC 2616: HTTP/1.1, Section 14: Header Field Definitions
Located at Env.inc.php