The attacker has sent PHP code in the HTTP request body, and he is trying to have that code executed by your web server.
The php://input references the request body (ie POST data). The auto_prepend_file directive allows the script to include PHP code in the same way that include() and require() work. If successful, the uploaded code would be prepended and executed.
The payload most likely contains a backdoor script and some code to call home to let the developer know that a hack was successful.
This is most likely a bot that has randomly selected your server, as opposed to a human manually attempting it.