http://ar.php.net/en/language.variables.predefined
$_REQUEST
Variables provided to the script via the GET, POST, and COOKIE input mechanisms, and which therefore cannot be trusted. The presence and order of variable inclusion in this array is defined according to the PHP variables_order configuration directive. This array has no direct analogue in versions of PHP prior to 4.1.0. See also import_request_variables().
Variables que llegan por POST, GET y COOKIE inputs, y por lo tanto NO PUEDE SER CONFIABLE. El orden de variables está definido acorde a la configuracion de variables_order blablabla...