如何通过php在文件夹中存储时更改图像名称

i am working on a website, in which each user can upload multiple images..i am able to upload the image and storing in a folder but i have a problem when two different user have the same image name, as i am storing image in a single folder. In the database where ,i am storing information, serial no is the unique key..i want to rename the image with adding serial no. with the image name to make it unique...please help me..to do it..i am working on a website, in which each user c