Thats simple with a microcontroller in between. I. e. an arduino + Ethernet shield + rfid reader. Can send you some links later.
Jan
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Chat: http://hackerspace.sg/chat
http://www.seeedstudio.com/depot/arduino-uno-p-694.html?cPath=79_81
http://www.seeedstudio.com/depot/wiznet-ethernet-shield-w5100-p-518.html?cPath=101_102
http://www.seeedstudio.com/depot/125khz-rfid-module-wiegand-p-225.html
http://www.seeedstudio.com/depot/rfid-tag-combo-125khz-5-pcs-p-700.html
All you need to do is wait for the RFID Module to send you data,
process it and send off your post request.
Jan
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Chat: http://hackerspace.sg/chat