html - Self-closing tags in XML files

Question

<tag id="foo" />

I noticed that they work with PHP SimpleXML.

But all XML examples I found on the web close them the old way:

<tag id="foo"></tag>

Is there any reason why I should use the old method?

Answer 1

See the W3C specs for XML and XHTML:

• http://www.w3.org/TR/xml/#sec-starttags
• http://www.w3.org/TR/xhtml1/dtds.html#a_dtd_XHTML-1.0-Strict

It depends on the Element Type declaration

An element with no content is said to be empty. The representation of an empty element is either a start-tag immediately followed by an end-tag, or an empty-element tag.

but also

Empty-element tags may be used for any element which has no content, whether or not it is declared using the keyword EMPTY. For interoperability, the empty-element tag SHOULD be used, and SHOULD only be used, for elements which are declared EMPTY.

This means, when your DTD contains something like

<!ELEMENT img EMPTY>

you should use

<img/>

unless you have good reason to use

<img></img>

Note that SHOULD is defined in RFC2119 as

This word, or the adjective "RECOMMENDED", mean that there may exist valid reasons in particular circumstances to ignore a particular item, but the full implications must be understood and carefully weighed before choosing a different course.

If you are working with XML that does not have a DTD or Schema, you can also influence how the XML is serialized with a predefined libxml constant:

LIBXML_NOEMPTYTAG (integer): Expand empty tags (e.g. <br/> to <br></br>)

But note that this option is currently just available in the functions DOMDocument::save and DOMDocument::saveXML, so you cannot use it with SimpleXml.