Дебаг приложения на PHP. Как улучшить код и избавиться от предупреждений?

class MYSQLi_DB {

	var $connection;

	function MYSQL_DB() {		
		$this->connection = mysqli_connect(SQL_SERVER, SQL_USER, SQL_PASS, SQL_DB) or die('Connect Error (' . mysqli_connect_errno() . ') '. mysqli_connect_error());
	}

function sitterLogin($username, $password) {
		$q = "SELECT sit1,sit2 FROM " . TB_PREFIX . "users WHERE username = '$username' AND access != " . BANNED;
		$result = mysqli_query($this->connection, $q);
		$dbarray = mysqli_fetch_array($result, MYSQLI_BOTH); //ОШИБКА ЗДЕСЬ
		if($dbarray['sit1'] != 0) {
			$q2 = "SELECT password FROM " . TB_PREFIX . "users WHERE id = " . $dbarray['sit1'] . " AND access != " . BANNED;
			$result2 = mysqli_query($this->connection, $q2);
			$dbarray2 = mysqli_fetch_array($result2, MYSQLI_BOTH);
		}
		if($dbarray['sit2'] != 0) {
				$q3 = "SELECT password FROM " . TB_PREFIX . "users WHERE id = " . $dbarray['sit2'] . " AND access != " . BANNED;
				$result3 = mysqli_query($this->connection, $q3);
				$dbarray3 = mysqli_fetch_array($result3, MYSQLI_BOTH);
		}
		if($dbarray['sit1'] != 0 || $dbarray['sit2'] != 0) {
			if($dbarray2['password'] == md5($password) || $dbarray3['password'] == md5($password)) {
				return true;
			} else {
				return false;
			}
		} else {
			return false;
		}
	}