In response to this question, I'm sure many people would object that PHP isn't the best choice for desktop programming, and would recommend something like Ruby or Python instead. I thought it would be interesting to have a discussion on why.
Aside from arguments about whether PHP is a good language overall, what are your criteria for a good desktop programming language (as opposed to web development)?
Performance is my criteria on desktop software. But I believe that desktop softwares will die someday :)
Is it installed by default on the target platform? Because if it is not, then there is more friction to adoption.
Because it is catered entirely toward web programming, and while it may have the necessary tools for desktop programming there are other languages which are much more capable, more efficient for the job.
Personally I don't like a lot of the language principles so I only use it when I'm forced to.
This sounds like a trite answer, but I think the reason most people don't use PHP as a desktop programming language is because people don't use PHP as a desktop programming language.
Meaning, there isn't a very strong community around GUI desktop applications using PHP - this will impact the tools a developer will have available, the resources he can go to for help, and for a company or commercial endeavour, the additional programmers that will have the skillsets necessary to work with a very niche set of tools.
Does $language_under_consideration do the job I want easier than $other_language?
E.g., I've found php interfaces to mysql to be great for quickie DB work. I'd not want to write php for a GUI app on Windows though....
PHP is designed specifically to create web applications. While it is technically possible to create desktop apps using it, you're really sanding off the corners to fit the square peg in the round hole.
There are so many programming languages designed with the desktop as a target, it just doesn't make sense to try and shoehorn PHP in to a world it's not meant for.
For perspective: PHP is my job
PHP for desktop applications:
The ease of distribution from a desktop application with the performance of a web application.
I'm a huge advocate of using PHP for quick development of web applications. It's easy to learn, has a huge community of other users that have solved most problems already.
However in addition to what other users have mentioned you may also want to consider the sheer number of vulnerabilities PHP already has (http://www.securityfocus.com/vulnerabilities).
If some of these things were addressed and perhaps packaged in something similar to what we see with Adobe AIR, there might be a good basis for desktop application production.
PHP is the new perl if you're going to use it as a programming language outside the web environment. If I'm going to be writing apps, I want a decient environmen to work in. Without 3rd party apps, debugging php programs can be a pain.
You say "Aside from arguments about whether PHP is a good language overall" -- but I think that is the crux of the issue. PHP became popular as a web development language because it integrates well with an existing stack of web tools (inline HTML, good Apache integration, built-in MySQL support, etc) -- not because it is well-designed language that is easy to work in. (It's not, by the way.)
If you take the web away and use PHP for desktop development, what do you have left? A weakly typed, inconsistently-featured language with no support for desktop UIs and a horrific mess of a standard library. People don't use PHP that way because it just doesn't make sense.
I'm surprised nobody has said this. As a web developer, I only use PHP because that's what the clients usually ask for. They want to deploy on any hosting they can find and all web hosting share one common denominator: PHP. Also, we do a lot of rescue mission style coding where we have to fix sites already written in PHP. As an honest developer can't recommend PHP as a platform in good conscience for serious web development. I would never choose PHP for any project, web or desktop. Python is pretty great for desktop apps. It's more dynamic than PHP, more sane, and more productive.
I don't see any difference in using PHP for web or desktop application, just different libraries. Qt and GTK+ are both availables to PHP, and if someone took the time to write such bindings it means more than someone is using it for desktop.
In my opinion, the success of PHP in the web is because there is no valid alternative: said in other words PHP is the less worse. In the desktop world it is not so lucky and it should compete with stronger contendents... I'll personally choose tons of other languages before PHP.