Processing as it stands requires access to a native windowing toolkit in order to create its parent Applet. If your webserver is a usual Unix based shared hosting setup, then this environment most likely will not have (or need) such a thing (it might not even have server side Java support) and hence you can't even launch your app in most cases. You could only run Processing if your webserver is Windows based or on Linux has the DISPLAY environment variable defined (pointing to the X-Windows device manager). To check if the latter is available, SSH into your webserver, type env and check if the DISPLAY var is listed...

Even if you can launch Processing, the other issues are: support of long running processes on shared hosts and the communication between PHP and your sketch on the server side. One approach would be to use PHP's exec function to launch your sketch and pass arguments. This will also require you to switch your Processing sketch to Java mode and implement a main() method in order to pick up these arguments... You can see an example on this wiki page:  http://wiki.processing.org/w/Setting_width/height_dynamically

An alternative approach I've been using just recently is more of an asynchronous solution and involves using the webservice infrastructure offered by Amazon:

* S3 to store parameter files uploaded from a local running app (e.g. Processing)

* SQS to provide a queue of render requests which are processed by...

* one or more EC2 instances with my render app polling the above queue, rendering highres images, uploading the results back to S3 and finally creating new blog posts on an external Wordpress installation.

The beauty of EC2 is that you create your disk images and install any software of your choice. It's your own virtual machine template and can be launched, frozen & torn down in no time... you'll only be charged for what you're using.

AWS has a fantastic Java API & documentation for all their services and you can handle all these communications with only a few lines of code... e.g. to upload a file to S3 is just this:

1. s3 = new AmazonS3Client(loginCreds);
2. s3.putObject(new PutObjectRequest("mybucket", "/foo/bar.txt", myLocalFile));

Hope that helps!