<?php

$a = array(
    'foo',
    'bar',
    array('baz', 'qux'),
    'one',
    'two',
    array('three'),
    'four',
);

$ri = new RecursiveIteratorIterator(new RecursiveArrayIterator($a));

var_dump(iterator_to_array($ri, false));

/**
array(8) {
  [0]=>
  string(3) "foo"
  [1]=>
  string(3) "bar"
  [2]=>
  string(3) "baz"
  [3]=>
  string(3) "qux"
  [4]=>
  string(3) "one"
  [5]=>
  string(3) "two"
  [6]=>
  string(5) "three"
  [7]=>
  string(4) "four"
}
*/