Robot follow my phone

First thing im have to sorry for my english.

i need to create a robot that can follow my application phone

Example i walk with my phone and robot following my phone all the time.

who have any idea?

sensor , example code

im so thank you for every comment.

Thank you.

Maybe this person can help: https://www.youtube.com/watch?v=KT1YiRLSOgQ

A little more detail.

https://www.youtube.com/watch?v=tAyWrJoVUbs