(PHP 4 >= 4.1.0, PHP 5, PHP 7)
$_FILES -- $HTTP_POST_FILES [deprecated] — HTTP File Upload variables
$HTTP_POST_FILES contains the same initial information, but is not a superglobal. (Note that $HTTP_POST_FILES and $_FILES are different variables and that PHP handles them as such)
This is a 'superglobal', or automatic global, variable. This simply means that it is available in all scopes throughout a script. There is no need to do global $variable; to access it within functions or methods.