Sql connect

class User {
protected $sqlConnection = NULL;

function __construct() {
GLOBAL $db;
$this->sqlConnection=$db;
}

public function AddUser() {
$sql=$this->sqlConnection;
$fieldValue = "test";
$queryInsert = "INSERT INTO users (nick) VALUES ( :nickname )";
$stmt = $sql->prepare($queryInsert);
$stmt->bindParam(":nickname", $fieldValue, PDO::PARAM_STR);

try {
$stmt->execute();
}
catch(PDOExecption $e) {
$sql->rollback();
print "Error!: " . $e->getMessage() . "</br>";
}
}

}

try {
$db = new PDO('mysql:host=localhost;dbname=adatbazisnev;charset=utf8', 'root', 'admin');
$db->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
$db->setAttribute(PDO::ATTR_EMULATE_PREPARES, false);
} catch(PDOException $ex) {
echo "An Error occured!";
}

$objUser = new User;
$objUser->AddUser();