$_SERVER

The $_SERVER variable is a special superglobal variable that holds information about the headers, scripts and path locations.

The entries in this array are created by the web server, therefore there is no guarantee that every web server provides any of these.

The $_SERVER variable is an associative array where the keys are the variable names and the values are the corresponding values.

$_SERVER is a superglobal: it is always available, in every scope.

<?php

echo $_SERVER['PHP_SELF'];

?>

Documentation

Related : $_POST, $_GET, $_FILES, $_COOKIE, $_ENV, $GLOBALS, HTTP Request Headers, Internet Protocol (IP)