Sorry I'm not familiar with php and I have to send something via Zend_Http_Client. How I can check that feature is available on server, because at the moment I've only got error 500.
You need to check if ZF is available in your project libraries. For Ubuntu, ZF should be in one of those:
or in your project directory.
In ZF "Zend_Http_Client", means that in your library directory you should have this folder structure: (...)/Zend/Http/Client/