SimpleXMLElement::__construct()

(PHP 5, PHP 7)

Creates a new SimpleXMLElement object

说明

Creates a new SimpleXMLElement object.

参数

$data

A well-formed XML string or the path or URL to an XML document if$data_is_urlis TRUE.

$options

Optionally used to specify additional Libxml parameters.

Note:

It may be necessary to pass LIBXML_PARSEHUGE to be able to process deeply nested XML or very large text nodes.

$data_is_url

By default,$data_is_urlis FALSE. Use TRUE to specify that$datais a path or URL to an XML document instead of string data.

$ns

Namespace prefix or URI.

$is_prefix

TRUE if$nsis a prefix,FALSE if it's a URI; defaults to FALSE.

返回值

Returns a SimpleXMLElement object representing$data.

错误／异常

Produces an E_WARNING error message for each error found in the XML data and additionally throws an Exception if the XML data could not be parsed.

Use libxml_use_internal_errors() to suppress all XML errors, and libxml_get_errors() to iterate over them afterwards.

范例

Note:

Listed examples may includeexample.php, which refers to the XML string found in the first example of the basic usage guide.

Example #1 Create a SimpleXMLElement object

<?php
include 'example.php';
$sxe = new SimpleXMLElement($xmlstr);
echo $sxe->movie[0]->title;
?>

以上例程会输出：

PHP: Behind the Parser

Example #2 Create a SimpleXMLElement object from a URL

<?php
$sxe = new SimpleXMLElement('http://example.org/document.xml', NULL, TRUE);
echo $sxe->asXML();
?>

更新日志

参见

• Basic SimpleXML usage
• simplexml_load_string()Interprets a string of XML into an object
• simplexml_load_file()Interprets an XML file into an object
• Dealing with XML errors
• libxml_use_internal_errors()Disable libxml errors and allow user to fetch error information as needed

You won't be able to load an XML file without root element:
//This will throw an exception
$xml = new SimpleXMLElement('<?xml version="1.0" encoding="UTF-8"?>', null, false);
//Here is the solution
$xml = new SimpleXMLElement('<?xml version="1.0" encoding="UTF-8"?><root></root>', null, false);

This class is extendable, but it's too bad that its constructor cannot be overriden (PHP says it's a final method). Thus the class should be wrapped using the delegation principle rather that extended.

The manual doesn't really explain what the $ns argument (and the accompanying $is_prefix) are for.
What they do is similar to the ->children() method: they set the context of the returned object to that namespace, so that access via ->elementName and ['attributeName'] refer to elements and attributes in that namespace.
In particular, they do *not* change the namespaces which exist on the document.
See this example:
<?php
// This XML contains two elements called <child>
// One is in the namespace http://example.com, with local prefix 'ws'
// The other has no namespace (no prefix, and no default namespace declared)
$xml = '<ws:example xmlns:ws="http://example.com"><child>Not in namespace</child><ws:child>In example namespace</ws:child></ws:example>';
$sx0 = new SimpleXMLElement($xml, 0, false);
$sx1 = new SimpleXMLElement($xml, 0, false, 'http://example.com');
$sx2 = new SimpleXMLElement($xml, 0, false, 'ws', true);
echo "
  Without: {$sx0->child}
  By namespace: {$sx1->child}
  By prefix: {$sx2->child}
";
?>
Output:
 Without: Not in namespace
 By namespace: In example namespace
 By prefix: In example namespace

Using libxml_use_internal_errors() may suppress errors but Exception still requires decent handling. I used following code snippet.
<?php
libxml_use_internal_errors(true);
try{
  $xmlToObject = new SimpleXMLElement($notSoWellFormedXML);
} catch (Exception $e){
  echo 'Please try again later...';
  exit();
}
?>

It's worth noting that the behavior of SimpleXmlElement::__construct is not exactly the same as simplexml_load_string, regarding libxml_use_internal_errors().
In my case,
<?php
libxml_use_internal_errors(true);
new \SimpleXmlElement($data);
?>
was still throwing error. But as soon as I switched to 
<?php
libxml_use_internal_errors(true);
simplexml_load_string($data);
?>
everything worked fine and I stopped getting an error.

It's worth noting, that you need to typecast dynamic property names to string in order to retrieve there value, for example:
<?php 
  $xml = new SimpleXMLElement('records.xml', 0, true);
  // This will work as expected because its a hard coded property value
  foreach($xml as $record){
    echo $record->id;  // Will output the ID fine
  }
  // Dynamic properties require typecasting to string
  $xml_field = 'id';
  foreach($xml as $record){
    // This will dump a SimpleXMLElement object
    var_dump($record->$xml_field);
    // This will output the value as expected
    echo (string)$record->$xml_field;
  }
?>

As I was filling out a bug report, I realized why (speculation here) the constructor is final: so that functions like simplexml_load_file and simplexml_load_string can work. I imagine the PHP-ized code looks something like
<?php
function simplexml_load_file($filename, $class_name = "SimpleXMLElement", $options = 0, $ns = "", $is_prefix = false) {
  return new $class_name($filename, $options, true, $ns, $is_prefix);
}
?>
If we were to use a different $class_name and change the constructor's definition these functions wouldn't work.
There's no easy, sensible solution that keeps simplexml_load_file and simplexml_load_string.