This doesn't really have a whole lot on the relevance of OO approaches, so it could be used either procedurally or object oriented.
The simplest changes to use multiple would be to take an array into the where condition and field conditions. Then you simply iterate them. Only really useful for constraining it as the types would all have to be AND conditions.
Something like this. With mysqli, you'd probably look at using prepared statements if the input is provided by users. That's a bit more of a pain in mysqli, its a piece of cake in PDO though.
if ($where && $field2)
$aCondition = array();
if (is_array($where) && is_array($field2))
// we'll just do a simple loop since its easy to read
$aWhere = array_combine($field2, $where);
foreach ($aWhere AS $key => $value)
$value = $this->data->mysqli->real_escape_string($value);
$aCondition = sprintf("%s=%s", $key, is_numeric($value) ? $value : "'$value'");
$value = $this->data->mysqli->real_escape_string($where);
$aCondition = sprintf("%s=%s", $field2, is_numeric($where) ? $where : "'$where'");
$result .= ' WHERE ' . implode(' AND ', $aCondition);
Oh yeah that's right. If you want more of an OOP approach, you would then provide it with class typehinting. The where condition would then become something like a ConnectionWhere class which is a class wrapper that can keep taking more and more conditions and just wrapping them up. This would allow you to provide AND and OR conditions without needing to fudge code like you would with the above.