File is a special kind of form data, when being uploaded to the server through HTTP POST request, PHP will create a $_FILES global array, the relevant file information will be stored in this global array. We will illustrate file upload with some code snippets using PHP and look into the internal work mechanism. Lastly will talk about file upload security.File uploadIn order for users to upload files in client side, we have to provide a form on the user interface. Since the uploaded file is a special data, it is different from other POST data, so we have have to set a special encoding for the f...