Variable global variable?

Question

I'm trying to automatically sanitize input on an array of global variables like so:

$sanitize = array('_POST', '_GET', '_REQUEST', '_COOKIE', '_SESSION');
foreach($sanitize as $type){
    $property = trim(strtolower($type), '_');
    $this->$property = $this->cleanse($$type);
}

But I get: Notice: Undefined variable: _REQUEST (and so on for all of the global variables I'm trying)

Is doing what I'm trying to accomplish actually possible?

Thanks.