$sum = function() {
	if(func_num_args() > 0) {
		$numbers = func_get_args();
	    $acc = 0;
	    foreach ($numbers as $n) {
	        $acc += $n;
	    }
	    return $acc;
	} else {
		return NULL;
	}
};

echo $sum(1, 2, 3, 4); // 결과값 : 10