XML 文件中的自闭合标签 [英] Self-closing tags in XML files

问题描述

<tag id="foo"/>

我注意到它们可以使用 PHP SimpleXML.

但是我在网上找到的所有 XML 示例都以旧方式关闭它们:

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

我有什么理由应该使用旧方法吗?

解决方案

查看 XML 和 XHTML 的 W3C 规范:

• 中定义为
  <块引用>

  这个词或形容词推荐"的意思是在特定情况下可能存在忽略特定项目，但必须了解全部含义并在选择不同的课程之前仔细权衡.

  如果您正在使用没有 DTD 或模式的 XML，您还可以使用 预定义的 libxml 常量:

  <块引用>

  LIBXML_NOEMPTYTAG(整数):将空标签(例如 <br/> 扩展为 <br></br>)

  但请注意，此选项目前仅在函数 DOMDocument::save 和 DOMDocument::saveXML 中可用，因此您不能将其与 SimpleXml 一起使用.

  <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?

  解决方案

  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.

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