Will I have to change my code to work with Idealstack?
Generally no, Idealstack maps AWS concepts to standard PHP so you don't have to worry about a lot of the 'hard problems' like shared sessions, shared filesystems or mail delivery. These use DynamoDB, EFS and SES without you needing to change your code to use them.
Sometimes for more 'fancy' frameworks though you may need to work around the framework's own attempts to implement things like sessions. In these cases there are generally modules you can use to implement native sessions (which is generally the best option - Idealstack uses DynamoDB for sessions which is in our experience the fastest, cheap and most scalable choice), or you can use one of the frameworks session handlers based on Redis/DynamoDB/Memcached or database sessions. We have documentation on the workarounds we recomend for different systems in the help section (if you use a framework that's not listed, get in touch and we'll figure it out for you, supporting all frameworks and CMS's is something we're pretty passionate about)