By default, t he superglobal variable $_REQUEST associative array is collection of contents of $_GET, $_POST and $_COOKIE variables. Settings in php.ini file decide composition of this variable. One of the directives in php.ini is request_order, which decides order in which PHP registers GET, POST and COOKIE variables. The presence and order of variables listed in this array is defined according to the PHP variables_order.
Also, if a PHP script is run fromm command line, the argc and argv variables are not included in $_REQUST array as their values are taken from $_SERVER array - which in turn is populated by web server.