Integrate Google Calender into Reminder App

I need to integrate Google Calendar into my php task reminder app. The app currently allows the user to create a reminder and have it delivered via phone using a text-to-speech engine. I would like to automate the reminder creation functions by allowing the user to sync their Google Cal with this app and have any calendar entries marked with a "keyword" to be automatically be added to the reminder app. A user control panel on the current reminder app will need to be created so the user can setup how early in advance they wish to be notified of calendar events. As this is an already functioning app you will need to integrate these features into the existing design and make use of the existing frameworks.