Please help me

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?

The only difference between your normal site and mobile site will be the CSS stylesheet that dictates the appearance of your webpage. None of your code will be different. PHP and MySQL will be fine for the backend.

In the ideal world the application code will not need to change based on the device. However, in the practical world there may be some cases where things need to be handled sightly differently application programming wise. That said, I would recommend at least beginning as Dan Grossman instructed. Most of the things that will need to change based on device will be client-side and more specifically CSS. To maximize efficiency and offer products that are both desktop and mobile “friendly” you should watch this. That presentation completely changed I way I thought of developing for mobile and desktop for that mater. At this point in time mobile is here to stay and so long as you have the budget and time there is really no reason to not program in a way that is adaptable to multiple devices. The primary reason for having separate desktop and mobile sites amounts to age. In reality though a well developed “mobile” version should be able to be transitioned to a desktop version, in theory.

Ok…Thanks a lot. Am using a simple CSS but am not sure yet if it going to be alright.
But since you have confirmed to me that the code is not going to affect anything, i think i can go on and just be careful with the CSS. However, i will let you know if i need any other assistance. Please if you have any other thing that you know could be of help to me for this project, please do let me know.
Many thanks