How to print $ _ FILE

How to print $ _ FILE is an ultra-Global Array. it should be printed on any php page, but it is indeed Array (). I don't know why,
So the question is, $ _ FILE must have any other conditions to print it out.
For example, I printed

Reply to discussion (solution)

Paste the form

I used print_r and var_dump to print out the array structure.

Print_r ($ _ FILES); echo output is not supported.

On which page did you submit it?And form

On which page did you submit it?And form

In fact, my goal is to print out the structure of the $ _ FILE array,

If no data is submitted, it cannot be printed. it is an empty array.