Create a simple php script for my SMS project

Hello, I want to create a simple php script on my server for my sms campagin. You must know how to write a simple STOP script (If someone reply stop will be removed and no more message will be sent in future to this number). Also If someone reply specific world they will get another sms with the message I want to be in the php file. If you know really vell php this should be not complex at all. It's really simple project.