This isn't exactly a question concerning Linux in particular, but has to do with the GNU GPL. In brief my question is this: If I've written a PHP web app which borrows GPL'd code, and I run it on a public server, am I required to release the source of my web app?

In terms of compiled languages, the GPL is pretty clear about this. If you release the binaries to the public, you must also release the source. But, when it comes to PHP, there aren't really any binaries in the first place. One possibility is that running the PHP script on a public web server is equivalent to publicly distributing binaries, but I'm not so sure that that's correct. Running a PHP script is probably closer to distributing the output of a binary, rather than distributing the binary itself. This makes me think that GPLing PHP code is rather useless. It seems that you'd never be required to release the source.

So, any GPL experts here know how this works with interpreted (eg PHP) languages?

GPL states that you have to make the source available aupon request. regardless of binary or non binray.

it is also ok to use 'snippets' of gpl'd code, and use a different license for the app as a whole though. (as long as you dont use LGPL'd code).

there are also ways to 'protect' php code (e.g. zend guard or obfuscators) yet if you release php code under GPL it makes no sense to use such methods.

I'm certainly not a an expert, nor am I a lawyer. So take this advice with caution.

But only if you distribute it and I think that's the key difference. In the case of a dynamic website the script/programs used are not distributed. Rather they are used to generate the html source. The html is viewed much like the letter you might produce from a GPL'd Word Processor. It's the output of the program

Even the FSF are a bit vague on this one. But I believe there are some changes in the GPL3 to address this.

If not releasing your changes when asked for is important to you then seek some finer legal clarification of the contract or at least get in contact with the Free Software Foundation for some clarification.

I'd be interested to hear about what you find out.

a website is a kind of making software published - and then also, if gpl'd and requested, you need to make the source available.

if you dont want to make your source available, you can simply utilize gpl'd software, but your are not required to make your website gpl'd and therefore are not requried to make source available