The AppendIterator class

joshdifabio is technically correct, but I don't see this as a bug. You can't rewind a generator and thats what append iterator does.
If you want to use AppendIterator with Generators just wrap them with NoRewindIterator:
<?php
function foo() {
 foreach ([] as $foo) {
 yield $foo;
 }
}
$append = new AppendIterator();
$append->append(new NoRewindIterator(foo()));
var_dump(iterator_to_array($append));
https://3v4l.org/pgiXB

Note that AppendIterator will segfault when iterating over an empty generator. Do not use AppendIterator in conjunction with generators.
https://3v4l.org/YC68k
https://bugs.php.net/bug.php?id=71436 

In many cases, especially for streaming sources, Generators are way more efficient. I noticed that the AppendIterator buffers the entire "inner iterator".
<?php
/**
 * This appends $next iterator to $iterator.
 */
function append_iterators(...$iterators){
 foreach($iterators as $iterator)
 foreach($iterator as $row)
 yield($row);
}
/**
 * Merge iterator takes one first from each iterator until
 * every iterator is empty.
 */
function merge_iterators(....$its) {
 $numberOfIts = sizeof($its);
 while($numberOfIts > 0) {
 $iterator = array_shift($its);
 yield($iterator->current());
 $iterator->next();
 if($iterator->valid())
 $its[] = $iterator;
 else
 $numberOfIts--;
 }
});
?>