Parse XML Document recursive - php

Question

I have XML documents containing information of articles, that have a kind of hierarchy:
<?xml version="1.0" encoding="UTF-8"?>
<page>
<elements>
<element>
<type>article</type>
<id>1</id>
<parentContainerID>page</parentContainerID>
<parentContainerType>page</parentContainerType>
</element>
<element>
<type>article</type>
<id>2</id>
<parentContainerID>1</parentContainerID>
<parentContainerType>article</parentContainerType>
</element>
<element>
<type>photo</type>
<id>3</id>
<parentContainerID>2</parentContainerID>
<parentContainerType>article</parentContainerType>
</element>
<... more elements ..>
</elements>
</page>
The element has the node parentContainerID and the node parentContainerType. If parentContainerType == page, this is the master element. The parentContainerID shows what's the element's master. So it should look like: 1 <- 2 <- 3
Now I need to build a new page (html) of this stuff that looks like this:
content of ID 1, content of ID 2, content of ID 3 (the IDs are not ongoing).
I guess this could be done with a recursive function. But I have no idea how to manage this?

Answer 1

Here is no nesting/recursion in the XML. The <element/> nodes are siblings. To build the parent child relations I would suggest looping over the XML and building two arrays. One for the relations and one referencing the elements.
$xml = file_get_contents('php://stdin');
$document = new DOMDocument();
$document->loadXml($xml);
$xpath = new DOMXpath($document);
$relations = [];
$elements = [];
foreach ($xpath->evaluate('//element') as $element) {
$id = (int)$xpath->evaluate('string(id)', $element);
$parentId = (int)$xpath->evaluate('string(parentContainerID)', $element);
$relations[$parentId][] = $id;
$elements[$id] = $element;
}
var_dump($relations);
Output:
array(3) {
[0]=>
array(1) {
[0]=>
int(1)
}
[1]=>
array(1) {
[0]=>
int(2)
}
[2]=>
array(1) {
[0]=>
int(3)
}
}
The relations array now contains the child ids for any parent, elements without a parent are in index 0. This allows you use a recursive function access the elements as a tree.
function traverse(
int $parentId, callable $callback, array $elements, array $relations, $level = -1
) {
if ($elements[$parentId]) {
$callback($elements[$parentId], $parentId, $level);
}
if (isset($relations[$parentId]) && is_array($relations[$parentId])) {
foreach ($relations[$parentId] as $childId) {
traverse($childId, $callback, $elements, $relations, ++$level);
}
}
}
This executes the callback for each node. The proper implementation for this would be a RecursiveIterator but the function should do for the example.
traverse(
0,
function(DOMNode $element, int $id, int $level) use ($xpath) {
echo str_repeat(' ', $level);
echo $id, ": ", $xpath->evaluate('string(type)', $element), "\n";
},
$elements,
$relations
);
Output:
1: article
2: article
3: photo
Notice that the $xpath object is provided as context to the callback. Because the $elements array contains the original nodes, you can use Xpath expression to fetch detailed data from the DOM related to the current element node.