$_SERVER is a predefined
'super global' variable.
$_SERVER contains information such as headers, paths, and script locations. The entries in this array are created by the web server. There is no guarantee that every web server will provide any of these; servers may omit some, or provide others not listed here.
PHP providing some Super Global variables which are an associative array and available in all scopes on script.
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<?php echo"<pre>"; print_r($_SERVER); ?>
Following are the information which can be retrieved using $_SERVER.