I just found this topic in a WriteRoom Google group which mentions the same "could not get a sandbox extension" error: https://groups.google.com/forum/#!msg/writeroomapp/GqBs6X1d8g0/EVhJZBgy1CcJ

This post is interesting:

If you open Console.app, do you an error containing this message when you try to save?

" -[NSFileCoordinator itemAtURL:willMoveToURL:] could not get a sandbox extension. "

If so, then the OS X sandbox is preventing WR from using that extension to save to and has decided that only *.txt or whatever will do. I'm not seeing *.md or *.mmd getting renamed on my end, but I'm having that problem with other extensions like *.org. 

It might be because I have several other Markdown editors installed, which is placating the sandbox regarding those extensions. Or something. In 10.8 there's a reference to a "related items" feature, but I actually am unable to find the documentation for it at all beyond a message from an Apple employee in October 2012 saying they were still working on the documentation for it.
That doesn't solve your problem, and I'm sorry about that as well as how long it took to get a reply up. 

I highlighted the interesting part. It sounds logical that another app might have messed up something in regards to file extensions that is preventing the sandboxing feature of the OS to change the file extension.

Thanks a lot for the screencast!

I have done exactly the same as you did in the video. I even disabled iCloud because it didn't appear as an option in your save panel so you seem to have it disabled. 

I have typed the same characters and used the same menu items, but it works just fine on my iMac and on my Macbook Pro.

Could you try to use "Repair permissions" in Disk Utility? Maybe there is indeed a permission problem somewhere on your hard disk.

I don't know if that's too much to ask, but: if you would be able to record the problem in a short screencast, that would be terrific!

So, it would be:

1. Open Textastic

2. Press CMD + N to get a new window

3. Type some code

4. Press CMD + S to save and save it as test.html on the Desktop

5. Now add some new code to the already written ..

6. Try to save again

I'm not sure at which point you save it as test.txt and when it then gets moved to test.html.

Do you have iCloud enabled on your Mac? I do.

Did you maybe save it under a file name that contains any special characters? Do you have any other apps running that might access newly created files? 

I tried to follow the steps on my machine and it worked without a problem. Which Mac OS X version do you use? I tried it on 10.8.3.

You can ask the iTunes team for a refund. There's a guide for this at http://gizmodo.com/5886683/how-to-get-a-refund-from-the-app-store

Textastic offers syntax highlighting for many languages including PHP, but it doesn't include compilers or interpreters due to restrictions of iOS.

If you try to preview the php file locally, only the html part will be interpreted by the web view. The PHP part will be ignored.

In order to preview PHP in Textastic, you can upload the file to a test server, open the preview screen, 
and enter the remote url of your test server.