Let MySql do it for you, by adding a column of type TIMESTAMP. If you will be adding reviews only, that's all you have to do. If you allows users to update/edit their review, you'll have to decide if they get a new timestamp each time they're updated. If they do, again there's nothing to do, but if you want to retain the timestamp column as the original date of creation you'll need to rewrite the existing value back to the column.
That is, in your INSERT query you won't mention the timestamp column (thus MySql will give it the default value consisting of the current time.) But when doing and UPDATE, you will need to have retrieved the value of timestamp column, and then say
UPDATE .... set .... created='$created' ...
Otherwise if it's omitted from the column list the value will be updated to the current time when the row is updated.