DOMDocument::getElementById()

(PHP 5, PHP 7)

Searches for an element with a certain id

说明

publicDOMDocument::getElementById(string $elementId): DOMElement

This function is similar to DOMDocument::getElementsByTagName but searches for an element with a given id.

For this function to work, you will need either to set some ID attributes with DOMElement::setIdAttribute or a DTD which defines an attribute to be of type ID. In the later case, you will need to validate your document with DOMDocument::validate or DOMDocument::$validateOnParse before using this function.

参数

$elementId

The unique id value for an element.

返回值

Returns the DOMElement or NULL if the element is not found.

范例

Example #1 DOMDocument::getElementById()Example

以下范例使用了book.xml，其内容如下：

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE books [
  <!ELEMENT books   (book+)>
  <!ELEMENT book    (title, author+, xhtml:blurb?)>
  <!ELEMENT title   (#PCDATA)>
  <!ELEMENT blurb   (#PCDATA)>
  <!ELEMENT author  (#PCDATA)>
  <!ATTLIST books   xmlns        CDATA  #IMPLIED>
  <!ATTLIST books   xmlns:xhtml  CDATA  #IMPLIED>
  <!ATTLIST book    id           ID     #IMPLIED>
  <!ATTLIST author  email        CDATA  #IMPLIED>
]>
<?xml-stylesheet type="text/xsl" href="style.xsl"?>
<books xmlns="http://books.php/" xmlns:xhtml="http://www.w3.org/1999/xhtml">
  <book id="php-basics">
    <title>PHP Basics</title>
    <author email="jim.smith@basics.php">Jim Smith</author>
    <author email="jane.smith@basics.php">Jane Smith</author>
    <xhtml:blurb><![CDATA[
<p><em>PHP Basics</em> provides an introduction to PHP.</p>
]]></xhtml:blurb>
  </book>
  <book id="php-advanced">
    <title>PHP Advanced Programming</title>
    <author email="jon.doe@advanced.php">Jon Doe</author>
  </book>
</books>

<?php
$doc = new DomDocument;
// We need to validate our document before refering to the id
$doc->validateOnParse = true;
$doc->Load('book.xml');
echo "The element whose id is 'php-basics' is: " . $doc->getElementById('php-basics')->tagName . "\n";
?>

以上例程会输出：

The element whose id is 'php-basics' is: book 

参见

• DOMDocument::getElementsByTagName() Searches for all elements with given local tag name

Please note that if your HTML does not contain a doctype declaration, then getElementById will always return null.

Had some issues with getElementById() while searching for a specific element in a XHTML document.
I wrote a small function what was solving my problem:
<?php
function getElementById($id)
{
  $xpath = new DOMXPath($this->domDocument);
  return $xpath->query("//*[@id='$id']")->item(0);
}
?>

From my experience, getElementById seem to work fine without any setups if you have loaded a HTML document. But in order for getElementById to work with a simple XML document that you've "constructed", you have to set up the id with "xml:" prefix and use setIdAttribute on the element you created or it won't work. See following example, hope this will save someone's frustration. If you have loaded the xml file, then all you have to make sure is the ID has a xml: prefix for the attribute. But if you start to append the XML document, don't forget to setIdAttribute on the id name or those elements or getElementById will return null when you try to find them. 
<?php
  /* test.xml
  <?xml version="1.0" encoding="utf-8"?>
  <root>
    <child xml:id="id_xxxxxx" status="partial">
      <sub_child>Some Data</sub_child>
    </child>
  </root>
  */
  $xmlDom = new DOMDocument('1.0', 'utf-8');
  $xmlDom->formatOutput = true;            // we want a nice output
  // create a root
  $eltRoot = $xmlDom->createElement("root");
  $xmlDom->appendChild($eltRoot);
  $eltChild = $xmlDom->createElement("child");
  $eltRoot->appendChild($eltChild);
  
  // add a id attribute
  $attr = $xmlDom->createAttribute("xml:id");  // needs xml prefix or getElementById won't work
  $eltChild->appendChild($attr);
  
  /// create the text node and append to the created element
  $tNode = $xmlDom->createTextNode("id_8120528");
  $attr->appendChild($tNode);
  $eltChild->setIdAttribute("xml:id", true);  // VERY IMPORT or getElementById won't work
  
  // add a id attribute
  $attr = $xmlDom->createAttribute("status");
  $eltChild->appendChild($attr);
  
  /// create the text node and append to the created element
  $tNode = $xmlDom->createTextNode("partial");
  $attr->appendChild($tNode);  
  
  // add a subchild
  $eltSub = $xmlDom->createElement("sub_child");
  $eltChild->appendChild($eltSub);
  
  $tNode = $xmlDom->createTextNode("Some Data");
  $eltSub->appendChild($tNode);
  $id = null;
  $id = $xmlDom->getElementById("id_8120528");
  
  assert ($id != null);
  
  $strId = $id->getAttribute("xml:id"); // bug? empty
  $strStatus = $id->getAttribute("status"); // this works!
  
  assert ($id !=null);
  
  $xmlDom->save("./_data/test.xml");
  
  $xmlDom->load("./_data/test.xml"); // reloading fixes the problem
  
  $nodeRoot = $xmlDom->getElementsByTagName("root");
  if ($nodeRoot->length > 0) {
    $eltRoot = $nodeRoot->item(0);  
  }
  
  assert($eltRoot != null);
  
  $id = null;
  $id = $xmlDom->getElementById("id_8120528");
  assert ($id != null);
  $strId = $id->getAttribute("xml:id"); // this works now!
  $strStatus = $id->getAttribute("status"); // this works!
    
  
?>

SAVE YOURSELF A MAJOR HEADACHE AND A LOT OF SEARCHING THROUGH DOCUMENTATION - 
Instead of using $object->setAttribute('id', 'id_name_here')
USE THIS: $object->setAttribute('xml:id', 'id_name_here')
Then, to get the node value: $domDocumentObject->getElementById('id_name_here');
The xml:id attribute should AUTOMATICALLY be defined!!
Woohoo! That was easy......

Validating a document from a DTD so as to use getElementById is sometimes impossible (for example when the head and body elements are not included yet in a XHtml document : the validation failed).
Fortunately, xml:id is supported by this function :)
That may be useful.
http://www.w3.org/TR/xml-id/

If you're trying to use getElementById with a xml file validated on a xsd file you must first use the schemaValidate function or getElementById will return null
Example:
 $dom = new DomDocument();
 $dom->load("users.xml");
 $dom->schemaValidate("users.xsd");
 $curruser = $dom->getElementById($user->name);

It seems getElementById works fine without setting validateOnParse to true. Which is nice since setting this to true caused some performance problems with my script.

If your XML document does not have a DTD that defines the "id" attribute as an ID, then the easiest thing to do is to use XPath->query() to find an element that matches "//[@id='x']"

You don't want to use "xml:id" ?
Here is the relaxNG trick (with a generic schema):
(tested with libxml 2.6.26)
<?php
$doc = new DOMDocument();
$doc->load(...);
$rng = '
<grammar xmlns="http://relaxng.org/ns/structure/1.0" datatypeLibrary="http://www.w3.org/2001/XMLSchema-datatypes">
  <start>
    <element>
      <anyName/>
      <ref name="anythingID"/>
    </element>
  </start>
  <define name="anythingID">
    <zeroOrMore>
      <choice>
        <element>
          <anyName/>
          <ref name="anythingID"/>
        </element>
        <attribute name="id">
          <data type="ID"/>
        </attribute>
        <zeroOrMore>
          <attribute><anyName/></attribute>
        </zeroOrMore>
        <text/>
      </choice>
    </zeroOrMore>
  </define>
</grammar>
';
$doc->relaxNGValidateSource($rng);
var_dump($doc->getElementById('id1'));
?>
Note that ID values must be valid ones :
 - integers do no work! 
 - @see http://www.w3.org/TR/REC-xml/#id
 - => (Letter | '_' | ':') ( Letter | Digit | '.' | '-' | '_' | ':' | CombiningChar | Extender )*