$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);

Index array
Array
(
  [PARA] => Array
    (
      [0] => 0
      [1] => 2
    )

  [NOTE] => Array
    (
      [0] => 1
    )
)

Vals array
Array
(
  [0] => Array
    (
      [tag] => PARA
      [type] => open
      [level] => 1
    )

  [1] => Array
    (
      [tag] => NOTE
      [type] => complete
      [level] => 2
      [value] => simple note
    )

  [2] => Array
    (
      [tag] => PARA
      [type] => close
      [level] => 1
    )
)

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

$p = xml_parser_create();
xml_parse_into_struct($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);
 

$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]['value'];
  if(empty($value)) continue;

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

$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 == 'para') {
  $index++;
  $result[$index] = array();
 }
}

function endElement($parser, $name) {
 global $result, $index, $currData;
 $name = strtolower($name);
 if($name == 'note' || $name == 'extra') {
  $result[$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);

$string = <<<XML
<?xml version='1.0'?>
<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);

$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();

$xmlstring = <<<XML
<?xml version='1.0'?>
<document>
 <cmd attr='default'>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;
}