XML count elements, If id exists increment by one - php

Question

What i am trying to do is count the elements under the root element. Then check if one id on that same level has the id value. When this occurs it needs to increment by one.
The code
public function _generate_id()
{
$id = 0;
$xpath = new DOMXPath($this->_dom);
do{
$id++;
} while($xpath->query("/*/*[#id=$id]"));
return $id;
}
example xml
<?xml version="1.0"?>
<catalog>
<book id="0">
<author>Gambardella, Matthew</author>
<title>XML Developer's Guide</title>
<genre>Computer</genre>
<price>44.95</price>
<publish_date>2000-10-01</publish_date>
<description>An in-depth look at creating applications
with XML.</description>
</book>
<book id="1">
<author>Ralls, Kim</author>
<title>Midnight Rain</title>
<genre>Fantasy</genre>
<price>5.95</price>
<publish_date>2000-12-16</publish_date>
<description>A former architect battles corporate zombies,
an evil sorceress, and her own childhood to become queen
of the world.</description>
</book>
</catalog>

Answer 1

You can use the following xpath query to get the maximum value of the id attribute:
$result = $xpath->query('/*/*[not(../*/#id > #id)]/#id');
In your function you can return this value incremented by 1:
return intval($result->item(0)->nodeValue) + 1;
Update: You can do the increment operation using XPath as well. Note DOMXPath::evaluate():
return $xpath->evaluate('/*/*[not(../*/#id > #id)]/#id + 1');
|------- +1 in xpath
This will give you 2 - but as a double. I would suggest to convert to integer before returning the result:
return (integer) $xpath->evaluate('/*/*[not(../*/#id > #id)]/#id + 1');

Answer 2

I suggest you create an array of all existing ID values first (which is a single xpath query) and then you check against it:
$id = 0;
while(isset($ids[$id])) {
$id++;
}
echo $id; # 2
Creating such a list is trivial running the xpath on SimpleXML, however this can be easily ported to DOMXPath as well with iterator_to_array:
<?php
$buffer = <<<BUFFER
<?xml version="1.0"?>
<catalog>
<book id="0">
<author>Gambardella, Matthew</author>
<title>XML Developer's Guide</title>
<genre>Computer</genre>
<price>44.95</price>
<publish_date>2000-10-01</publish_date>
<description>An in-depth look at creating applications
with XML.</description>
</book>
<book id="1">
<author>Ralls, Kim</author>
<title>Midnight Rain</title>
<genre>Fantasy</genre>
<price>5.95</price>
<publish_date>2000-12-16</publish_date>
<description>A former architect battles corporate zombies,
an evil sorceress, and her own childhood to become queen
of the world.</description>
</book>
</catalog>
BUFFER;
$xml = simplexml_load_string($buffer);
$ids = array_flip(array_map('intval', $xml->xpath("/*/*/#id")));
Interactive Demo
Additionally I suggest you to not use 0 (zero) as ID value.

Answer 3

Use simplexml, try this
$xml = simplexml_load_string($this->_dom);
$id = is_array($xml->book) ? $xml->book[count($xml->book)-1]->attributes()->id : 0;
return $id;