During a workshop we were discussing the concept of a Data Transfer Object (DTO). The main characteristic of a DTO is that it holds only primitive-type values (strings, integers, booleans), lists or maps of these values including “nested” DTOs. Not sure who came up with this idea, but I’m using it because it ensures that the DTO becomes a data structure that only enforces a schema (field names, the expected types, required fields, and optional fields), but doesn’t enforce semantics for any value put into it. That way it can be created from any data source, like submitted form values, CLI arguments, JSON, XML, Yaml, and so on. Using primitive values in a DTO makes it clear that the values are not validated. The DTO is just used to transfer or carry data from one layer to the next. A question that popped up during the workshop: can we consider DateTimeImmutable a primitive-type value too? If so, can we use this type inside DTOs?

In essence, everything is a string.

Well, you can always go one layer deeper and find out what a string really is, but for web apps I work on, both input data and output data are strings. The input is an HTTP request, which is a plain-text message that gets passed to the web server, the PHP server, the framework, and finally a user-land controller. The output is an HTTP response, which is also a plain-text message that gets passed to the client. If my app needs the database to load or store some data, that data too is in its initial form a string. It needs to be deserialized into objects to do something and later be serialized into strings so we can store the results.