Home security system remotely control alarm system

I have search few website and cant find any coding for this project. What i suggest is a system with Arduino, Sim900 GSM/GPRS, buzzer and PIR sensor. The function is like this when the PIR sensor sense movement the GSM will send a alert sms to user mobile phone than depends on the user, if the user reply "ACTIVATE" than arduino will trigger the buzzer, if the user reply" DISARM" than buzzer stop. If already have this coding please attach here for me. I need it as fast as possible. Advance thanx..