Well a php file (that isn't a library, so one that produces output) is one of the things that you could write to be a lambda function. But not every lambda function can be expressed as a php file. So I could download some video filtering lambda functions and they can be written in anything, then I can link them against my application, which I write in PHP.
The language/platform/dependency freedom is what takes it from literal CGI script to lambda function.
It took a moment of thinking to decide which of "literal CGI script" or "lambda function" you were attributing "language/platform/dependency freedom" to.
Ah, that's fair. I was mostly attempting to highlight that a php script is a subset of lambda functions (from a conceptual viewpoint). Of course it's also a subset of CGI scripts as well (in a much more literal viewpoint).
This does not sound right. The platform independence and freedom is the big win for lambda where it is basically the perfect vendor lock-in? With serverless you give up a lot of independence.
The language/platform/dependency freedom is what takes it from literal CGI script to lambda function.