Description:
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When loading an XML document from memory (string, file, whatever), such as via 
DOMDocument::loadXml(), PHP tells libxml to use the current working directory, 
or I think sometimes the script's folder path, as the base URI. libxml (and 
PHP's thin API) expose this value as the baseURI property in the DOM, overriding 
it only as needed when xml:base is in scope.

As pointed out in the comments on Bug #44367, neither PHP nor libxml claim to 
support DOM Core Level 3, so maybe this PHP/libxml "baseURI" property shouldn't 
be expected to behave as it would in a L3-compliant DOM. Nevertheless, I think 
it's reasonable to expect that something called "baseURI" would in fact be 
formatted as a URI, thus making it useful as an actual base URI for resolving 
URI references like href attribute values in XHTML documents and Atom feeds.

So, my request is that the path be prefixed with "file://", at the very least. 
This will have the added benefit of making things forward-compatible with DOM 
Core L3, if such support is intended for the future.

Also, my experience with XML apps suggests that a more typical default would be 
a file URI corresponding to the script itself, rather than the cwd or script 
folder. This would have no effect on resolution of relative references (except 
an empty string), so it seems harmless in that regard. However, I would 
understand if there's a desire to avoid disclosing the script's file name in the 
DOM.

Test script:
---------------
<?php
$xml_string = "<greeting>hello world</greeting>";
$doc = new DOMDocument();
$doc->loadXml($xml_string);
$document_base = $doc->baseURI;
$document_element_base = $doc->documentElement->baseURI;
echo "  DOMDocument->baseURI = " . $document_base;
// A URI, as opposed to a URI reference, is always absolute.
// That is, it has a scheme/protocol (something before ":").
echo (strpos($document_base, ":") == false ? " (not a real URI!)" : " (probably OK!)") . "\n";
echo "  DocumentElement->baseURI = " . $document_element_base;
echo (strpos($document_element_base, ":") == false ? " (not a real URI!)" : " (probably OK!)") . "\n";
?>

Expected result:
----------------
  DOMDocument->baseURI = file:///usr/home/x/ (probably OK!)
  DocumentElement->baseURI = file:///usr/home/x/ (probably OK!)

Actual result:
--------------
  DOMDocument->baseURI = /usr/home/x/ (not a real URI!)
  DocumentElement->baseURI = /usr/home/x/ (not a real URI!)