Have a look at the PEAR HTTP package. It allows for POST.
All you need is to establish a connection and send headers and data according to the RFC standards for HTTP.
However, your question talks about GET. When you say 'sending' do you mean from the browser?
Therefore:
1. Answer is no. You cannot send POST variables from the browser without a form.
2. Answer is yes, if you mean if a PHP script can send POST requests. Then it doesn't matter where the script runs, it will always be on a server that executed PHP.