This is not unrealistic. There are reasons that hold above and beyond the regular plans like tying into the low-level APIs in the case of Adobe AIR.
It is also not something that Java offers you to soothe out your preferences to build your web across multiple platforms reliably. This also superimposes the use of Python as a scripting language. So, what's the catch? With PHP, you will be able to work more independently and plan focally towards desktop presence to develop better cross-platform apps representing your individual business needs. Furthermore, you don't need a low-level OS API access rather you just need to build your applications with PHP. The environment presents to a novice user helps him further go about the things more confidently and with more resources by his side to back up the entire effort.
Among many suitable ways to develop a cross-platform app with PHP, here are the top three that are really good at providing you with a middle ground between easy syntax and good structure. Have a look!
Powered by PHP 5.5.x, Nightrain is a packager written in Python that uses PHP's internal server to host your app. Nightrain is compatible with different PHP apps/frameworks. Well, doing that all it requires is you opt for SQLite in the database. For the rest of the things, you can tweak in the bundle provided by php.ini and get the extensions that are not available otherwise.
Nightrain is still in the process of development so you can find advanced aspects and features missing which could keep you from building bigger and complex applications. One more thing that adds to the downside is Windows app is launched after a command prompt which can be difficult for technically weak users to go with.
The good news is the browser object is being worked on and is expected to get better soon. This will fix many critical limitations that it has currently which will lead to a great experience working with Nightrain.
Call it wxPHP or wxWidgets for PHP just one and the same thing. Technically, it could be referred to as a PHP extension that embraces the whole wxWidgets library to allow developers to write multi-platform desktop applications using native graphical components with different platforms. Install it as a separate program where applications are simply loaded as files and easily send them across elsewhere with ease. Using wxPHP you are not building a website you do otherwise, rather you are bringing together different wx widgets from the library and putting them to work for you, working online. wxPHP helps you create your cross-platform desktop app with the least technicalities involved and doing least experiments.
TideSDK works differently than the other two as it allows you to install SDKs to build your application considering the prerequisites of each platform separately. Once you set up the application, it would arrange for all the necessary helpers to help you proceed with development and put you at ease with distribution by allowing you to do the bundling of applications. The best part is you can simply build your apps via the helper app or through the command line. No matter which way you choose to build your app, you would be able to distribute it in a purely executable format. You can alternatively distribute it as an installable package that gets a complete app treatment (with an ideal installation process integrated), allowing you to deal with different package managers to effectively handle the program.