Multidimensional array iteration

You could use an Iterator to iterate over the array and then produce your desired output:

class TranformArrayIterator extends RecursiveIteratorIterator
{
    protected function indent()
    {
        echo str_repeat("\t", $this->getDepth());
        return $this;
    }
    public function beginIteration()
    {
        echo '<nodes>', PHP_EOL;
    }
    public function endIteration()
    {
        echo '</nodes>', PHP_EOL;
    }
    public function beginChildren()
    {
        $this->indent()->beginIteration();
    }
    public function endChildren()
    {
        $this->indent()->endIteration();
    }
    public function current()
    {
        return sprintf('%s<node>%s</node>%s',
                       str_repeat("\t", $this->getDepth() +1),
                       parent::current(),
                       PHP_EOL);
    }
}

and then assemble it like this:

$iterator = new TranformArrayIterator(new RecursiveArrayIterator($nodes));

foreach($iterator as $val) {
    echo $val;
}

outputs

<nodes>
        <node>parent node</node>
        <node>parent node</node>
        <nodes>
                <node>child node</node>
                <node>child node</node>
                <nodes>
                        <node>grand child node</node>
                        <node>grand child node</node>
                </nodes>
        </nodes>
</nodes>

To blank out $key when using $key => $val, add this to TraverseArrayIterator

public function key()
{ 
    return '';
}

Since your aim seems to be to produce XML, you could also pass an XMLWriter as a collaborator to the Iterator. This allows for more control over the generated XML and also makes sure the output is valid XML:

class TranformArrayIterator extends RecursiveIteratorIterator
{
    private $xmlWriter;

    public function __construct(
        XmlWriter $xmlWriter, 
        Traversable $iterator, 
        $mode = RecursiveIteratorIterator::LEAVES_ONLY , 
        $flags = 0)
    {
        $this->xmlWriter = $xmlWriter;
        parent::__construct($iterator, $mode, $flags);
    }

    public function beginIteration()
    {
        $this->xmlWriter->startDocument('1.0', 'utf-8');
        $this->beginChildren();
    }
    public function endIteration()
    {
        $this->xmlWriter->endDocument();
    }
    public function beginChildren()
    {
        $this->xmlWriter->startElement('nodes');
    }
    public function endChildren()
    {
        $this->xmlWriter->endElement();
    }
    public function current()
    {
        $this->xmlWriter->writeElement('node', parent::current());
    }
}

You'd then use it like this:

$xmlWriter = new XmlWriter;
$xmlWriter->openUri('php://output');
$xmlWriter->setIndent(true);
$xmlWriter->setIndentString("\t");
$iterator = new TranformArrayIterator(
    $xmlWriter,
    new RecursiveArrayIterator($nodes)
);

and foreach'ing over it will produce the same output then (but adding the XML prolog)

<?php

$nodes = array(
    "parent node",
    "parent node",
    array(
        "child node",
        "child node",
        array(
            "grand child node",
            "grand child node"
        )
    )
);

$s = '<node>';
$arr = $nodes;

while(count($arr) > 0)
{
    $n = array_shift($arr);
    if(is_array($n))
    {
        array_unshift($arr, null);
        $arr = array_merge($n, $arr);
        $s .= '<node>';
    }
    elseif(is_null($n))
        $s .= '</node>';
    else
        $s .= '<node>'.$n.'</node>';
}
$s .= '</node>';

echo $s;

?>