I think this feature is partly duplicates code of valueiterator: .../include/valueiterator.h .../zend/hashiterator.h.
Now the same can be done like so:

std::vector<Php::Value> keys;
keys.reserve(array.size());
for(auto &it: array)
{
    keys.push_back(it.first);
}

The HashIterator has a bug, if the array has a reference item, for(auto &it: array) will break the array. for example:

Php::Value getKeys(Php::Parameters &params) {
    Php::Value array = params[0];
    std::vector<Php::Value> keys;
    keys.reserve(array.size());
    for(auto &it: array)
    {
        keys.push_back(it.first);
    }
}

<?php
   $a = array();
   $a[] = &$a;
   $a[] = "hello";
   var_dump($a);
   var_dump(getKeys($a));
   var_dump($a);
?>

the result is:

array(2) {
  [0]=>
  &array(2) {
    [0]=>
    *RECURSION*
    [1]=>
    string(5) "hello"
  }
  [1]=>
  string(5) "hello"
}
array(2) {
  [0]=>
  int(0)
  [1]=>
  int(1)
}
NULL

and when use iterator on an object Value, we can only get the public properties. But the keys method can get all properties(include private and protected properites).