I need help

I have a school project a head of me:
I have been told to design and build a web-based application basically for quiz/test maker where the student will also be able to answer and submit.
I have also been told to consider mobile devices and computers.
My question is:
Do i need to write a different code when considering mobile devices or the code i will write for normal website would work for mobile devices?
the tools i want to use is PHP and MySQL…hope this will work fine?