For those of you who have had the opportunity of writing web applications in PHP and then as an application server (eg. Python-based solutions like CherryPy or Pylons), in what context are application servers a better alternative to PHP?
I tend to favor PHP simply because it's available on just about any web server (especially shared host), but I'm looking for other good reasons to make an informed choice. Thank you.
There are several products in PHP which fill the same space as CherryPy or Pylons.
(except, of course, they don't run Python ;)
Have a look at -
Personally, I prefer Drupal, which works as a great framework and comes with a lot of CMS and community site features out of the box. The ones above are quite different in many ways, but any of these should offer you the best of both worlds if you want an app framework / appserver that runs on PHP.
Which one is the right choice is largely a matter of taste, although each has its various advantages and disadvantages.
There are many more - these are just the ones I've heard good things about from colleagues and collaborators. It's not a complete list.
The advantage of deployment for PHP is a huge one. You will also have a large developer base, and sound like you already have considerable expertise. In general if you know how to use a programming language well, you are safer to stick with it.
The advantages of using a Python web framework like Pylons are that your code will be neater, and more maintainable. Of course this is possible with PHP, but seems much harder to achieve. Python is simply a nicer language than PHP, so that might affect your decision.
Personally, I would not describe either Pylons or CherryPy as an "application server", if you want a Python application server, try Zope. (They both do serve WSGI Applications, but that is a topic for another question.) There seem to be plenty of equivalent frameworks for PHP, and they have been listed in other answers.
Using application servers like Pylons, Django, etc. require much more work to setup and deploy then PHP applications which are generally supported out of the box. I run a few Django apps and had to learn a bit of configuring apache with mod_python in order to get things to work. I put forth the effort because coding in python is much more enjoyable to me than PHP and after you get the Apache config right once you never really have to mess with it again. On another note, if you decide to go with a framework like Django, Rails, Pylons, .... they tend to solve a lot of small repetitive tasks that you would otherwise do on your own. But frameworks are their own huge topic of discussion.
Python web-apps tend to require more initial setup and development than the equivalent PHP site (particularly so for small sites). There also tend to be more reusable pieces for PHP (ie Wordpress as a blog). Configuring a server to run Python web-apps can be a difficult process, and not always well documented. PHP tends to be very easy to get running with Apache.
Also, as PHP is very widely used and is heavily used by beginners, there tends to be very good documentation for it.
However, Python is much more fun, and much more maintainable. It scales well (in development complexity terms, rather than traffic).
Personally, I would also say that using Python tends to train you to solve problems in a better way. I am definitely a better developer for having learned the Pythonic way of doing things.
I have a feeling that some of the responses didn't address the initial question directly, so I decided to post my own. I understand that the question was about the difference between the mod_php deployment model and the application server deployment model.
In simple words, PHP executes a given script on every request, and the application has no knowledge of what has happened before (unless it is emulated somehow). Moreover even the source code is being parsed on every request (unless you use a bytecode cache like APC). This process can be slow, especially if you have a framework with complex initialization.
In contrast to this, the application server has to be started once, and then it waits for a request to be processed. The application server should clean up resources after every requests (allocated memory, open descriptors, etc.), it can also pool certain resources (like database connections) that can be reused between requests for extra performance.
This later model (application server) is more efficient in most cases, but on the other hand more difficult to setup and maintain. It is also more demanding, as you have to pay more attention to the resources you utilize, in order to avoid resource leaks.