How to make arduino(ethernet shield) always listen to HTTP request from PHP

Hi all..

I'm on my way developing a simple full-duplex communication between Arduino and PHP(server side scripting). Here are the objectives :

1) Open/closed arduino's PIN controlled from Server side(simple php page). - STUCKED 2) Received POST/GET request from Arduino to insert temperature values into database. This is simple as detected the temperature value from the sensor and at the end making HTTP request to server side - DONE

What i mean by full-duplex is, arduino can receive/send request from/to web server running on top of PHP language, and at the same time, server side also can receive/send request from/to arudino as well.

What i cant figure out is, how the arduino keep listening HTTP request made by server side??

The web server and arduino are in the same network and the web server hosted inside local PC running on lampp(linux mint). The ethernet shield connected via cat5 cable through router directly.

What is the CORE concept to make both of its communicated?