Question

I’m trying to understand what specifically $key => $tmp_name is doing in the following code:

if(isset($_FILES['files']))
{
    foreach($_FILES['files']['tmp_name'] as $key => $tmp_name)
    {
        echo $_FILES['files']['name'][$key], "\n";
        move_uploaded_file($tmp_name, 'img/'.$_FILES['files']['name'][$key]);
    }
}

Answer 1

This is how you access your values if you have an upload form that has multiple fields, all named files[] or files[x] where x is a number:

example html:

<input type="file" name="files[]" />    // first field
<input type="file" name="files[]" />    // ...
<input type="file" name="files[]" />    // ...

The confusing thing is that the sent-in values are not grouped by index, but by their property (error, tmp_name, etc.).

Edit: Based on your edit, I would recommend reading up on foreach and arrays in general.