理解 PHP 代码这个 xml2json 实现包括三部分：
- xml2json.php —— 这个 PHP 类包括两个静态函数
- xml2json_test.php —— 执行xml2json 转换函数的测试驱动程序
- test1.xml、test2.xml、test3.xml、test4.xml —— 复杂程度不同的 XML 文件
为了简化起见，本文省略了代码中的详细注释。不过后面附的源文件中包含完整的注释。要了解完全的程序逻辑细节，请参阅所附的源文件（请参阅）。
定义了一些要用到的常量。第一行代码导入了 Services_JSON 实现。
清单 4. 定义 xml2json.php 中的常量1
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| require_once 'json/JSON.php';
// Internal program-specific Debug option.
define ("DEBUG", false);
// Maximum Recursion Depth that we can allow.
define ("MAX_RECURSION_DEPTH_ALLOWED", 25);
// An empty string
define ("EMPTY_STR", "");
// SimpleXMLElement object property name for attributes
define ("SIMPLE_XML_ELEMENT_OBJECT_PROPERTY_FOR_ATTRIBUTES", "@attributes");
// SimpleXMLElement object name.
define ("SIMPLE_XML_ELEMENT_PHP_CLASS", "SimpleXMLElement");
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中的代码片段是 xml2json 转换器的入口函数。它接收 XML 数据作为输入，将 XML 字符串转化成 SimpleXMLElement 对象，然后发送给该类的另一个（递归）函数作为输入。这个函数将 XML 元素转化成 PHP 关联数组。这个数组再被传给 Services_JSON 编码器作为其输入，该编码器给出 JSON 格式的输出。
清单 5. 使用 xml2json.php 中的 Services_JSON1
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| public static function transformXmlStringToJson($xmlStringContents) {
$simpleXmlElementObject = simplexml_load_string($xmlStringContents);
<br>
if ($simpleXmlElementObject == null) {
return(EMPTY_STR);
}
<br>
$jsonOutput = EMPTY_STR;
<br>
// Let us convert the XML structure into PHP array structure.
$array1 = xml2json::convertSimpleXmlElementObjectIntoArray($simpleXmlElementObject);
<br>
if (($array1 != null) && (sizeof($array1) > 0)) {
// Create a new instance of Services_JSON
$json = new Services_JSON();
// Let us now convert it to JSON formatted data.
$jsonOutput = $json->encode($array1);
} // End of if (($array1 != null) && (sizeof($array1) > 0))
<br>
return($jsonOutput);
} // End of function transformXmlStringToJson
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中这段长长的代码片段采用了 PHP 开放源码社区（请参阅）提出的递归技术。它接收输入的 SimpleXMLElement 对象，沿着嵌套的 XML 树递归遍历。将访问过的 XML 元素保存在 PHP 关联数组中。可以通过修改 中定义的常量来改变最大递归深度。
清单 6. xml2json.php 中的转换逻辑1
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| public static function convertSimpleXmlElementObjectIntoArray($simpleXmlElementObject,
&$recursionDepth=0) {
// Keep an eye on how deeply we are involved in recursion.
<br>
if ($recursionDepth > MAX_RECURSION_DEPTH_ALLOWED) {
// Fatal error. Exit now.
return(null);
}
<br>
if ($recursionDepth == 0) {
if (get_class($simpleXmlElementObject) != SIMPLE_XML_ELEMENT_PHP_CLASS) {
// If the external caller doesn't call this function initially
// with a SimpleXMLElement object, return now.
return(null);
} else {
// Store the original SimpleXmlElementObject sent by the caller.
// We will need it at the very end when we return from here for good.
$callerProvidedSimpleXmlElementObject = $simpleXmlElementObject;
}
} // End of if ($recursionDepth == 0) {
<br>
if (get_class($simpleXmlElementObject) == SIMPLE_XML_ELEMENT_PHP_CLASS) {
// Get a copy of the simpleXmlElementObject
$copyOfsimpleXmlElementObject = $simpleXmlElementObject;
// Get the object variables in the SimpleXmlElement object for us to iterate.
$simpleXmlElementObject = get_object_vars($simpleXmlElementObject);
}
<br>
// It needs to be an array of object variables.
if (is_array($simpleXmlElementObject)) {
// Is the array size 0? Then, we reached the rare CDATA text if any.
if (count($simpleXmlElementObject) <= 0) {
// Let us return the lonely CDATA. It could even be
// an empty element or just filled with whitespaces.
return (trim(strval($copyOfsimpleXmlElementObject)));
}
<br>
// Let us walk through the child elements now.
foreach($simpleXmlElementObject as $key=>$value) {
// When this block of code is commented, XML attributes will be
// added to the result array.
// Uncomment the following block of code if XML attributes are
// NOT required to be returned as part of the result array.
/*
if($key == SIMPLE_XML_ELEMENT_OBJECT_PROPERTY_FOR_ATTRIBUTES) {
continue;
}
*/
<br>
// Let us recursively process the current element we just visited.
// Increase the recursion depth by one.
$recursionDepth++;
$resultArray[$key] =
xml2json::convertSimpleXmlElementObjectIntoArray($value, $recursionDepth);
<br>
// Decrease the recursion depth by one.
$recursionDepth--;
} // End of foreach($simpleXmlElementObject as $key=>$value) {
<br>
if ($recursionDepth == 0) {
// That is it. We are heading to the exit now.
// Set the XML root element name as the root [top-level] key of
// the associative array that we are going to return to the caller of this
// recursive function.
$tempArray = $resultArray;
$resultArray = array();
$resultArray[$callerProvidedSimpleXmlElementObject->getName()] = $tempArray;
}
<br>
return ($resultArray);
} else {
// We are now looking at either the XML attribute text or
// the text between the XML tags.
return (trim(strval($simpleXmlElementObject)));
} // End of else
} // End of function convertSimpleXmlElementObjectIntoArray.
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成功遍历 XML 树之后，该函数就用 PHP 关联数组转换和存储了所有的 XML 元素（根元素和所有的孩子元素）。复杂的 XML 文档，得到的 PHP 数组也同样复杂。一旦 PHP 数组构造完成，Services_JSON 编码器就很容易将其转化成 JSON 格式的数据了。要了解其中的递归逻辑，请参阅存档的源文件。 |
