$db->query($Consulta)or die(mysqli_error('sis_archivos'));

$db->query($Consulta)or die($db->error);

<?php
function register($card, $name, $surname, $email, $passwd) /*public function in a class*/
{
	$mysqli = new mysqli("localhost", "root", "toor", "database");
	
	/* Check connection */
	if ($mysqli->connect_errno)
	{
		die("An error occurred trying to connect to the database. (" . $mysqli->connect_error . ').'); // // Show MySQL errors only for debug.
	}
	
	$sql = $mysqli->query("SELECT * FROM `users` WHERE `card` = '$card'");
	//$register = $mysqli->recorrer($sql); // This function does not exist (see num_rows below).
	//$name = $name . " " . $surname; // Disable because the query below save both in different columns.
	
	if ($sql->num_rows == 0) // Check if the I.D. card exists.
	{
		$sql = "INSERT INTO `users` (card, name, surname, email, passwd, job, image) "
				. "VALUES ('$card', '$name', '$surname', '$email', '$passwd', '', '')";
				
		$mysqli->query($sql) or die("An error occurred trying to do the query. (" . $mysqli->error . ').'); // Show MySQL errors only for debug.
		
		echo ("* User added successfully.");
	}
	else
	{
		echo ("* An user already exists with your I.D. card.");
	}
	
	$mysqli->close();
}
?>

register("MyCard", "John", "Smith", "[email protected]", "p4ssw0rd");