How can I upload files in PHP?

I need to implement file uploads in my PHP application, allowing users to upload files such as images or documents. What's the best way to handle file uploads in PHP? Can someone provide a step-by-step guide or code example on how to create a file upload form, validate uploaded files, and move them to a secure location on the server? Additionally, how can I prevent security vulnerabilities such as file upload exploits or denial-of-service attacks when handling file uploads?