It is beyond the scope of this book to explore all the information exchanged between server and client when a request is made, not least because PHP handles most of these details for you. You should gain a basic understanding of this process, however, especially if you intend to write scripts that fetch Web pages or check the status of Web addresses.
HTTP stands for Hypertext Transfer Protocol. It is essentially a set of rules that defines the process by which a client sends a request and a server returns a response. Both client and server provide information about themselves and the data to be transferred. Much of this information becomes available to you in superglobal arrays.