使用php查找正在上传的文件的扩展名

I am using PHP for a website. I have to get users upload their pics. Now, the pics can be in jpeg or png or gif format. My problem is that i identify a user by a unique userid, so I want to save the pics as: userid.jpg or userid.png etc. Now, how do i find out the extension of the file selected by the user using php?I am using PHP for a website. I have to get use