Mixing processing instructions and element nodes gives inconsistent results in SimpleXML

Description

The following code:

<?php

$xml = <<<XML
<?xml version="1.0" encoding="utf-8"?>
<container>
    <x/><?hello world?>
</container>
XML;

$sxe = simplexml_load_string($xml);

var_dump($sxe->children());

// x and hello swapped
$xml = <<<XML
<?xml version="1.0" encoding="utf-8"?>
<container>
    <?hello world?><x/>
</container>
XML;

$sxe = simplexml_load_string($xml);

var_dump($sxe->children());

Resulted in this output:

object(SimpleXMLElement)#2 (2) {
  ["x"]=>
  object(SimpleXMLElement)#4 (0) {
  }
  ["hello"]=>
  object(SimpleXMLElement)#5 (0) {
  }
}
object(SimpleXMLElement)#1 (1) {
  ["x"]=>
  object(SimpleXMLElement)#5 (0) {
  }
}

But I expected this output instead:

object(SimpleXMLElement)#2 (2) {
  ["x"]=>
  object(SimpleXMLElement)#4 (0) {
  }
  ["hello"]=>
  object(SimpleXMLElement)#5 (0) {
  }
}
object(SimpleXMLElement)#1 (1) {
  ["hello"]=>
  object(SimpleXMLElement)#%d (0) {
  }
  ["x"]=>
  object(SimpleXMLElement)#%d (0) {
  }
}

PHP Version

PHP 8.1+

Operating System

Linux

Start with the two PHP reproductions in the issue and inspect the SimpleXML implementation and its existing tests for children() ordering. Compare processing-instruction and element-node handling in both input orders, then add coverage showing the expected order and run the relevant SimpleXML tests.