Proposal: Support declare(function_and_const_lookup = 'global')

[TysonAndre]

RFC - https://wiki.php.net/rfc/use_global_elements

This adds an implementation and tests for my proposal in
https://news-web.php.net/php.internals/107877
and https://externals.io/message/107953
(those links has background information and why I'm proposing this.
I created this PR to prepare for creating an RFC document,
and demonstrate that this is doable)

• This PR is deliberately making this behavior opt-in (e.g. not emitting notices or changing the behavior of existing code, such as unmaintained third-party libraries)

Normally, for a non-fully-qualified function or constant name,
PHP would check for My\NS\my_function() and My\NS\MY_CONST
at runtime before checking the global namespace (for every usage).

declare(function_and_const_lookup = global) would be the same as fully qualifying every
function call or constant usage, and avoids the second check.

This would be used as follows:
(No implicit uses are added - sprintf should be resolved in the same way in the below example
whether the function was declared in the same file or a different file)

declare(function_and_const_lookup = global);
 
namespace MyNS;

// Outside the global namespace, without 'use function MyNS\sprintf',
// declaring a function is a compile error when function_and_const_lookup='global'.
// PHP already only warns about the name already
// being in use if the namespace being referred to is different.
// It does not emit notices, warnings, or errors if the namespace is the same.
use function MyNS\factorial;
use function MyNS\sprintf;
// Outside the global namespace, without 'use const MyNS\MY_CONST',
// declaring a constant is a compile error when function_and_const_lookup='global'.
// PHP already only warns about the name already
// being in use if the namespace being referred to is different.
// It does not emit notices, warnings, or errors if the namespace is the same.
use const MyNS\MY_PREFIX;
 
const MY_PREFIX = 'Prefix';
 
function sprintf($msg, ...$args) {
    // this forces the implementer to explicitly refer to sprintf from the global namespace.
    return \sprintf(MY_PREFIX . " $msg", ...$args);
}
 
function factorial(int $n) {
    return $n > 1 ? factorial($n - 1) * $n : 1;
}

// The below function and constant references now unambiguously refer to the global namespace.
//
// Without "declare(function_and_const_lookup='global') ...",
// php would have checked for MyNS\version_compare
// and MyNS\PHP_VERSION at runtime before checking the global namespace.
// This can now be optimized to a constant by opcache, to eliminate dead code.
if (version_compare(PHP_VERSION, '8.0.5') >= 0) {
    // ...
}