我需要为每个子域单独的文件和数据库

Question

i have made a web application in php and mysql. It is actually an online test software. I have hosted it on www.vstudy.in. Now anyone signing up on the page can give test. But i also provide coaching institutes online test software. Like for a coaching name xyz i create a subdomain xyz.vstudy.in and host files there with their seperate database. i am providing the test portal to 4 institutes. Now if i change a file i have to change it on all the subdomains.It isn't a big deal initially but as my clients will grow it can become hectic also. Do i need to continue it in the way i am doing or there is any easy way out to manage things so that i don't need to manage files on all the subdomains. Please help ??