Description:
------------
when using a function that is exported by php (ZEND_API) it's not possible to call the function directly. You've to create a reference to the function and then it is possible to call it.

i would expect that it is possible to call the function directly.

these are working fine:
$a = $zend->zend_write; $a("aaa", 3);
($zend->zend_write)("aaa", 3);
(clone $zend->zend_write)("aaa", 3);

this doesn't work -> Attempt to call undefined C function 'zend_write'
$zend->zend_write("aaa", 3);
$a = [$zend, 'zend_write']; $a("aaa", 3);

Test script:
---------------
    public static function assertObSame(string $expected, callable $cb)
    {
        ob_start();
        $cb();
        $res = ob_get_contents();
        ob_end_clean();

        static::assertSame($expected, $res);
    }

    public function testReturnU64()
    {

        $zend = FFI::cdef("
    typedef int (*zend_write_func_t)(const char *str, size_t str_length);
    extern zend_write_func_t zend_write;
        ");

        static::assertObSame('aaa', function() use ($zend) { $a = $zend->zend_write; $a("aaa", 3); });
        static::assertObSame('aaa', function() use ($zend) { ($zend->zend_write)("aaa", 3); });
        static::assertObSame('aaa', function() use ($zend) { (clone $zend->zend_write)("aaa", 3); });

        // does not work -> end up in a Attempt to call undefined C function 'zend_write'
        static::assertObSame('aaa', function() use ($zend) { $zend->zend_write("aaa", 3); });
        static::assertObSame('aaa', function() use ($zend) { $a = [$zend, 'zend_write']; $a("aaa", 3); });

    }

Expected result:
----------------
all ways to call zend_write should give the same result (pass the assertion).

Actual result:
--------------
FFI\Exception: Attempt to call undefined C function 'zend_write'