This post relates to the implications of a few of these titles.
If you consider yourself one of the below and feel misrepresented, please do not be offended! These are based on my own perception and experiences. Please feel free to comment on the article to tell me I am wrong.
PHP developerFor developers at a similar age to myself, it is often a developer's first language after HTML. This is because it is/was cheap/free to get some hosting that allows PHP to be run. They can make a simple website and want to make it more dynamic. Still using PHP implies the developer has not ventured out of this comfort zone.
Also, the ability to hack something together in a script (coupled with the fact that the language has horrible inconsistencies) means a PHP developer will often be perceived as someone who can put something simple together without much thought given to architecture or testability.
While there are some PHP projects like this (not at DM!), it is possible to write good PHP. Even Facebook (not exactly small-fry) is partly still using PHP.
Development can be test-driven with PHPunit, and there are frameworks (such as Yii) which are a great way to enforce MVC. I hope that kids who are picking up a bit of PHP will find Yii or similar, and will learn a bit about MVC (though better still, they could learn web programming with rails or something).
Java developerWhile the language is pretty old now (and is a bit dated in some respects) the Java ecosystem is thriving. Its open nature has led to the availability of many libraries and frameworks. Additionally, the Java ecosystem is not just limited to the Java language. Scala, groovy, fantom, clojure and many more.
If someone introduced themselves as a Java developer, I would assume they had an understanding of architecture that I might not assume a PHP developer had. I would assume they had the capacity to produce more testable, maintainable code. Most importantly, due to the number of libraries and frameworks available, I would assume a Java developer had experience in to identifying what available tools best suit the task at hand (albeit still within the Java-ecosystem).
.NET developerWhile .NET is a framework rather than a language, I do not often hear people introduce themselves as a C# developer or VB developer. A key difference between Microsoft's approach and the Java ecosystem is Microsoft has historically hidden the implementation from the developer ("The framework takes care of that for you!") while in Java-world you have more control.
In my view, a .NET developer has experience with the framework which means they can put something together to do the task at hand in the Microsoft-standard manner. The project will be something that another .NET developer can look at and understand.
Ruby developerRuby got very popular with Ruby on Rails, so I would expect a Ruby developer to be a web developer with a firm grasp of MVC. Due to the relative youth of Ruby's popularity, I would expect a Ruby developer to have worked in an environment where using newer technology is more important than established solutions, so they are likely to be aware of what is going on in the software industry.