This is a 2-part project.
1. Write the app. Either in native Android/iOS/Windows language, a HTML5-app in a wrapper or a generic tool that generates working apps for the different phone OS'es.
2. Write the backend that runs on your server. Using PHP is an option, but there are others. A database might be essential, although that's not clear to me from your very short requirements list.
I am new to mobile app development but i would like to benefit from your experience in this field. I want to do a simple mobile application where one phone sends a notification to another phone based on security number. I asked someone and he told me to use php but I don't know how to start. I mean I don't know what to search for at the beginning to start. I really appreciate it if someone can help.