使用SimpleXmlElement创建RSS提要的xmlns命名空间重定义错误？

Question

I am trying to use PHP to create an XML document that will be used for an RSS feed. I've got it working for the most part, but I am getting a error on line 2 at column 29: redefinition of the xmlns prefix is forbidden while attempting to add an attribute with a namespace to my document.

I've tried the accepted answer here: Add rss xmlns namespace definition to a php simplexml document? but that changes the first line to <rss... instead of <xml...

Here is the code that I am working with:

<?php
$xml = new SimpleXMLElement('<xml vesion="1.0" />');
$rss = $xml->addChild('rss');
$rss->addAttribute('version','2.0');
$rss->addAttribute("xml:base",'http://intranet/bapm/rss-avd','xml');
$rss->addAttribute('xmlns:dc',"http://purl.org/dc/elements/1.1/",'xmlns');
....

the line that adds the xml:base attribute works fine, but when I add the line that adds the xmlns:dc attribute I get the error. If I don't add the 'xmlns' as the third argument, I get no errors, but the rendered attribute omits the namespace?

Answer 1

The error message:

redefinition of the xmlns prefix is forbidden

is because xmlns is defined in XML (all names starting with the letters XML regardless of case are reserved), so you can not use "xmlns" as prefix. However in your code you use it as prefix:

$rss->addAttribute('xmlns:dc', "http://purl.org/dc/elements/1.1/", 'xmlns');
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It is just not possible to add such attributes. Remove that line of code and the error goes away. Add the namespace declaration to the string when you create the new object, as shown in the accepted answer to "Add rss xmlns namespace definition to a php simplexml document?".

To make this chrystal clear: You can not use SimpleXMLElement::addAttribute() to make an XML namespace declaration.

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Edit: SimpleXML was originally not designed to handle XML Namespaces and not all functionality is easily accessible (or straight-forward). The probably most clean way is using the sister library DOMDocument ("DOM").

For a more detailed review how to add Namespace Declarations in SimpleXML please see the following existing Q&A material here on the Stackoverflow site:

• Unable add namespace with PHPs SimpleXML (October 2009)
• How to declare an XML namespace prefix with DOM/PHP? (August 2010)
• Unable to add Attribute with Namespace Prefix using PHP Simplexml (October 2011)