Four methods for php to parse xml

XML processing is often encountered in the development process, and PHP also has rich support for it. This article only briefly explains some of the parsing technologies, including: Xml parser, SimpleXML, XMLReader, DOMDocument.

1. XML Expat Parser:

XML Parser uses the Expat XML parser. Expat is an event-based parser that treats XML documents as a series of events. When an event occurs, it calls a specified function to handle it. Expat is a validation-free parser that ignores any DTD linked to the document. However, if the document is not in good form, it will end up with an error message. Because it is event-based and has no validation, Expat is fast and suitable for web applications.

The advantage of XML Parser is its good performance, because it does not load the entire xml document into memory and then process it, but processes it while parsing it. But precisely because of this, it is not suitable for those who need to dynamically adjust the XML structure or perform complex operations based on the XML context structure. If you just want to parse and process a well-structured xml document, then it can complete the task well. It should be noted that XML Parser only supports three encoding formats: US-ASCII, ISO-8859-1 and UTF-8. If your xml data is in other encodings, you need to convert it to one of the above three first.
There are generally two commonly used parsing methods for XML Parser (actually two functions): xml_parse_into_struct and xml_set_element_handler.

xml_parse_into_struct

This method parses the xml data into two arrays:
index array - contains a pointer to the location of the value in the Value array
value Array - contains data from the parsed XML

These two arrays are a bit cumbersome to describe textually, so let’s look at an example (from the official php documentation)

$simple = "<para><note>simple note</note></para>";
$p = xml_parser_create();
xml_parse_into_struct($p, $simple, $vals, $index);
xml_parser_free($p);
echo "Index array\n";
print_r($index);
echo "\nVals array\n";
print_r($vals);

Output:
Index array
Array
(
[PARA] => 🎜>
[NOTE] => Array
(
[0] => 1
)

)

Vals array
Array
(
[0] => Array

(

[tag] => PARA
[type] => open
[level] => 1
)

[1] => Array
(
[tag] => NOTE
[type] => complete

[level] => 2

                         ] => simple note
)

[2] => Array
(
) [tag] => PARA
[type] => close

[level] => 1

)
)

The index array uses the label name as key, and the corresponding value is an array, which includes the position of all this label in the value array. Then through this position, find the value corresponding to this label.

If the format of each set of data in xml is different and cannot be completely unified, then you should pay attention when writing code, you may get wrong results. For example, the following example:

If you traverse in the above way, the code seems simple, but there are hidden dangers. The most fatal thing is that you get the wrong result (extra3 runs into the second para) . So we need to traverse in a more rigorous way:

In fact, I rarely use the xml_parse_into_struct function, so if the so-called "rigorous" code above is not preserved, there will be bugs in other situations. - -|

$xml = '
<infos>
<para><note>note1</note><extra>extra1</extra></para>
<para><note>note2</note></para>
<para><note>note3</note><extra>extra3</extra></para>
</infos>
&#39;;

$p = xml_parser_create();
xml_parse_into_struct($p, $xml, $values, $tags);
xml_parser_free($p);
$result = array();
//下面的遍历方式有bug隐患
for ($i=0; $i<3; $i++) {
  $result[$i] = array();
  $result[$i]["note"] = $values[$tags["NOTE"][$i]]["value"];
  $result[$i]["extra"] = $values[$tags["EXTRA"][$i]]["value"];
}
print_r($result);

xml_set_element_handler

This method is to set the callback function for parser to handle the start and end of elements. Also included is the callback function xml_set_character_data_handler used to set data for the parser. The code written in this way is clearer and easier to maintain.

$result = array();
$paraTagIndexes = $tags['PARA'];
$paraCount = count($paraTagIndexes);
for($i = 0; $i < $paraCount; $i += 2) {
  $para = array();
  //遍历para标签对之间的所有值
  for($j = $paraTagIndexes[$i]; $j < $paraTagIndexes[$i+1]; $j++) {
    $value = $values[$j][&#39;value&#39;];
    if(empty($value)) continue;

    $tagname = strtolower($values[$j][&#39;tag&#39;]);
    if(in_array($tagname, array(&#39;note&#39;,&#39;extra&#39;))) {
      $para[$tagname] = $value;
    }
  }
  $result[] = $para;
}

Example:

It can be seen that although the set handler method has more lines of code, it has clear ideas and better readability, but its performance is slightly slower than the first method, and Not very flexible. XML Parser supports PHP4 and is suitable for systems using older versions. For PHP5 environment, give priority to the following method.

2. SimpleXML

$xml = <<<XML
<infos>
<para><note>note1</note><extra>extra1</extra></para>
<para><note>note2</note></para>
<para><note>note3</note><extra>extra3</extra></para>
</infos>
XML;

$result = array();
$index = -1;
$currData;

function charactor($parser, $data) {
  global $currData;
  $currData = $data;
}

function startElement($parser, $name, $attribs) {
  global $result, $index;
  $name = strtolower($name);
  if($name == &#39;para&#39;) {
    $index++;
    $result[$index] = array();
  }
}

function endElement($parser, $name) {
  global $result, $index, $currData;
  $name = strtolower($name);
  if($name == &#39;note&#39; || $name == &#39;extra&#39;) {
    $result[$index][$name] = $currData;
  }
}

$xml_parser = xml_parser_create();
xml_set_character_data_handler($xml_parser, "charactor");
xml_set_element_handler($xml_parser, "startElement", "endElement");
if (!xml_parse($xml_parser, $xml)) {
  echo "Error when parse xml: ";
  echo xml_error_string(xml_get_error_code($xml_parser));
}
xml_parser_free($xml_parser);

print_r($result);

SimpleXML是PHP5后提供的一套简单易用的xml工具集，可以把xml转换成方便处理的对象，也可以组织生成xml数据。不过它不适用于包含namespace的xml，而且要保证xml格式完整(well-formed)。它提供了三个方法：simplexml_import_dom、simplexml_load_file、simplexml_load_string，函数名很直观地说明了函数的作用。三个函数都返回SimpleXMLElement对象，数据的读取/添加都是通过SimpleXMLElement操作。

$string = <<<XML
<?xml version=&#39;1.0&#39;?>
<document>
  <cmd>login</cmd>
  <login>imdonkey</login>
</document>
XML;

$xml = simplexml_load_string($string);
print_r($xml);
$login = $xml->login;//这里返回的依然是个SimpleXMLElement对象
print_r($login);
$login = (string) $xml->login;//在做数据比较时，注意要先强制转换
print_r($login);

SimpleXML的优点是开发简单，缺点是它会将整个xml载入内存后再进行处理，所以在解析超多内容的xml文档时可能会力不从心。如果是读取小文件，而且xml中也不包含namespace，那SimpleXML是很好的选择。

3。 XMLReader

XMLReader也是PHP5之后的扩展（5.1后默认安装），它就像游标一样在文档流中移动，并在每个节点处停下来，操作起来很灵活。它提供了对输入的快速和非缓存的流式访问，可以读取流或文档，使用户从中提取数据，并跳过对应用程序没有意义的记录。
以一个利用google天气api获取信息的例子展示下XMLReader的使用，这里也只涉及到一小部分函数，更多还请参考官方文档。

$xml_uri = 'http://www.google.com/ig/api?weather=Beijing&hl=zh-cn';
$current = array();
$forecast = array();

$reader = new XMLReader();
$reader->open($xml_uri, 'gbk');
while ($reader->read()) {
  //get current data
  if ($reader->name == "current_conditions" && $reader->nodeType == XMLReader::ELEMENT) {
    while($reader->read() && $reader->name != "current_conditions") {
      $name = $reader->name;
      $value = $reader->getAttribute('data');
      $current[$name] = $value;
    }
  }

  //get forecast data
  if ($reader->name == "forecast_conditions" && $reader->nodeType == XMLReader::ELEMENT) {
    $sub_forecast = array();
    while($reader->read() && $reader->name != "forecast_conditions") {
      $name = $reader->name;
      $value = $reader->getAttribute('data');
      $sub_forecast[$name] = $value;
    }
    $forecast[] = $sub_forecast;
  }
}
$reader->close();

XMLReader和XML Parser类似，都是边读边操作，较大的差异在于SAX模型是一个“推送”模型，其中分析器将事件推到应用程序，在每次读取新节点时通知应用程序，而使用XmlReader的应用程序可以随意从读取器提取节点，可控性更好。
由于XMLReader基于libxml，所以有些函数要参考文档看看是否适用于你的libxml版本。

4。 DOMDocument

DOMDocument还是PHP5后推出的DOM扩展的一部分，可用来建立或解析html/xml，目前只支持utf-8编码。

$xmlstring = <<<XML
<?xml version=&#39;1.0&#39;?>
<document>
  <cmd attr=&#39;default&#39;>login</cmd>
  <login>imdonkey</login>
</document>
XML;

$dom = new DOMDocument();
$dom->loadXML($xmlstring);
print_r(getArray($dom->documentElement));

function getArray($node) {
  $array = false;

  if ($node->hasAttributes()) {
    foreach ($node->attributes as $attr) {
      $array[$attr->nodeName] = $attr->nodeValue;
    }
  }

  if ($node->hasChildNodes()) {
    if ($node->childNodes->length == 1) {
      $array[$node->firstChild->nodeName] = getArray($node->firstChild);
    } else {
      foreach ($node->childNodes as $childNode) {
      if ($childNode->nodeType != XML_TEXT_NODE) {
        $array[$childNode->nodeName][] = getArray($childNode);
      }
    }
  }
  } else {
    return $node->nodeValue;
  }
  return $array;
}

从函数名上看感觉跟JavaScript很像，应该是借鉴了一些吧。DOMDocument也是一次性将xml载入内存，所以内存问题同样需要注意。PHP提供了这么多的xml处理方式，开发人员在选择上就要花些时间了解，选择适合项目需求及系统环境、又便于维护的方法。

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