目前，JSON已经成为最流行的数据交换格式之一，各大网站的API几乎都支持它。
我写过一篇《数据类型和JSON格式》，探讨它的设计思想。今天，我想总结一下PHP语言对它的支持，这是开发互联网应用程序（特别是编写API）必须了解的知识。
从5.2版本开始，PHP原生提供json_encode()和json_decode()函数，前者用于编码，后者用于解码。
一、json_encode()
该函数主要用来将数组和对象，转换为json格式。先看一个数组转换的例子：
$arr = array ('a'=>1,'b'=>2,'c'=>3,'d'=>4,'e'=>5);
echo json_encode($arr);
结果为
{"a":1,"b":2,"c":3,"d":4,"e":5}
再看一个对象转换的例子：
$obj->body = 'another post'; $obj->id = 21; $obj->approved = true; $obj->favorite_count = 1; $obj->status = NULL; echo json_encode($obj);
结果为
{
"body":"another post",
"id":21,
"approved":true,
"favorite_count":1,
"status":null
}
由于json只接受utf-8编码的字符，所以json_encode()的参数必须是utf-8编码，否则会得到空字符或者null。当中文使用GB2312编码，或者外文使用ISO-8859-1编码的时候，这一点要特别注意。
二、索引数组和关联数组
PHP支持两种数组，一种是只保存"值"（value）的索引数组（indexed array），另一种是保存"名值对"（name/value）的关联数组（associative array）。
由于javascript不支持关联数组，所以json_encode()只将索引数组（indexed array）转为数组格式，而将关联数组（associative array）转为对象格式。
比如，现在有一个索引数组
$arr = Array('one', 'two', 'three');
echo json_encode($arr);
结果为：
["one","two","three"]
如果将它改为关联数组：
$arr = Array('1'=>'one', '2'=>'two', '3'=>'three');
echo json_encode($arr);
结果就变了：
{"1":"one","2":"two","3":"three"}
注意，数据格式从"[]"（数组）变成了"{}"（对象）。
如果你需要将"索引数组"强制转化成"对象"，可以这样写
json_encode( (object)$arr );
或者
json_encode ( $arr, JSON_FORCE_OBJECT );
三、类（class）的转换
下面是一个PHP的类：
class Foo {
const ERROR_CODE = '404';
public $public_ex = 'this is public';
private $private_ex = 'this is private!';
protected $protected_ex = 'this should be protected';
public function getErrorCode() {
return self::ERROR_CODE;
}
}
现在，对这个类的实例进行json转换：
$foo = new Foo; $foo_json = json_encode($foo); echo $foo_json;
输出结果是
{"public_ex":"this is public"}
可以看到，除了公开变量（public），其他东西（常量、私有变量、方法等等）都遗失了。
四、json_decode()
该函数用于将json文本转换为相应的PHP数据结构。下面是一个例子：
$json = '{"foo": 12345}';
$obj = json_decode($json);
print $obj->{'foo'}; // 12345
通常情况下，json_decode()总是返回一个PHP对象，而不是数组。比如：
$json = '{"a":1,"b":2,"c":3,"d":4,"e":5}';
var_dump(json_decode($json));
结果就是生成一个PHP对象：
object(stdClass)#1 (5) {
["a"] => int(1)
["b"] => int(2)
["c"] => int(3)
["d"] => int(4)
["e"] => int(5)
}
如果想要强制生成PHP关联数组，json_decode()需要加一个参数true：
$json = '{"a":1,"b":2,"c":3,"d":4,"e":5}';
var_dump(json_decode($json,true));
结果就生成了一个关联数组：
array(5) {
["a"] => int(1)
["b"] => int(2)
["c"] => int(3)
["d"] => int(4)
["e"] => int(5)
}
五、json_decode()的常见错误
下面三种json写法都是错的，你能看出错在哪里吗？
$bad_json = "{ 'bar': 'baz' }";
$bad_json = '{ bar: "baz" }';
$bad_json = '{ "bar": "baz", }';
对这三个字符串执行json_decode()都将返回null，并且报错。
第一个的错误是，json的分隔符（delimiter）只允许使用双引号，不能使用单引号。第二个的错误是，json名值对的"名"（冒号左边的部分），任何情况下都必须使用双引号。第三个的错误是，最后一个值之后不能添加逗号（trailing comma）。
另外，json只能用来表示对象（object）和数组（array），如果对一个字符串或数值使用json_decode()，将会返回null。
var_dump(json_decode("Hello World")); //null
下面给大家介绍哦php语言的json实现
由于开发一个ajax file manager for web开源项目,数据交换使用的json格式，后来发现在低版本的php上运行会有问题，仔细调试发现json_decode和json_encode无法正常工作，于是查阅资料，发现低版本的php没有实现这两个函数，为了兼容性，我只好自己实现一个php版的json编码解码代码，并保证和json2.js的一致，测试调试并通过，现在将其公布出来，供有相同需求的同学使用：
<?php
/* * ****************************************************************************
* $base: $
*
* $Author: $
* Berlin Qin
*
* $History: base.js $
* Berlin Qin // created
*
* $contacted
* webfmt@gmail.com
* www.webfmt.com
*
* *************************************************************************** */
/* ===========================================================================
* license
*
* 、Open Source Licenses
* webfmt is distributed under the GPL, LGPL and MPL open source licenses.
* This triple copyleft licensing model avoids incompatibility with other open source licenses.
* These Open Source licenses are specially indicated for:
* Integrating webfmt into Open Source software;
* Personal and educational use of webfmt;
* Integrating webfmt in commercial software,
* taking care of satisfying the Open Source licenses terms,
* while not able or interested on supporting webfmt and its development.
*
* 、Commercial License ? fbis source Closed Distribution License - CDL
* For many companies and products, Open Source licenses are not an option.
* This is why the fbis source Closed Distribution License (CDL) has been introduced.
* It is a non-copyleft license which gives companies complete freedom
* when integrating webfmt into their products and web sites.
* This license offers a very flexible way to integrate webfmt in your commercial application.
* These are the main advantages it offers over an Open Source license:
* Modifications and enhancements doesn't need to be released under an Open Source license;
* There is no need to distribute any Open Source license terms alongside with your product
* and no reference to it have to be done;
* No references to webfmt have to be done in any file distributed with your product;
* The source code of webfmt doesn't have to be distributed alongside with your product;
* You can remove any file from webfmt when integrating it with your product.
* The CDL is a lifetime license valid for all releases of webfmt published during
* and before the year following its purchase.
* It's valid for webfmt releases also. It includes year of personal e-mail support.
*
* ************************************************************************************************************************************************* */
function jsonDecode($json)
{
$result = array();
try
{
if (PHP_VERSION_ID > )
{
$result = (array) json_decode($json);
}
else
{
$json = str_replace(array("\\", "\""), array("&#;", "&#;"), $json);
$parts = preg_split("@("[^"]*")|([[]{},:])|s@is", $json, -, PREG_SPLIT_NO_EMPTY | PREG_SPLIT_DELIM_CAPTURE);
foreach ($parts as $index => $part)
{
if (strlen($part) == )
{
switch ($part)
{
case "[":
case "{":
$parts[$index] = "array(";
break;
case "]":
case "}":
$parts[$index] = ")";
break;
case ":":
$parts[$index] = "=>";
break;
case ",":
break;
default:
break;
}
}
}
$json = str_replace(array("&#;", "&#;", "$"), array("\\", "\"", "\$"), implode("", $parts));
$result = eval("return $json;");
}
}
catch (Exception $e)
{
$result = array("error" => $e->getCode());
}
return $result;
}
function valueTostr($val)
{
if (is_string($val))
{
$val = str_replace('"', "\"", $val);
$val = str_replace("\", "\\", $val);
$val = str_replace("/", "\/", $val);
$val = str_replace("t", "\t", $val);
$val = str_replace("n", "\n", $val);
$val = str_replace("r", "\r", $val);
$val = str_replace("b", "\b", $val);
$val = str_replace("f", "\f", $val);
return '"' . $val . '"';
}
elseif (is_int($val))
return sprintf('%d', $val);
elseif (is_float($val))
return sprintf('%F', $val);
elseif (is_bool($val))
return ($val ? 'true' : 'false');
else
return 'null';
}
function jsonEncode($arr)
{
$result = "{}";
try
{
if (PHP_VERSION_ID > )
{
$result = json_encode($arr);
}
else
{
$parts = array();
$is_list = false;
if (!is_array($arr))
{
$arr = (array) $arr;
}
$end = count($arr) - ;
if (count($arr) > )
{
if (is_numeric(key($arr)))
{
$result = "[";
for ($i = ; $i < count($arr); $i++)
{
if (is_array($arr[$i]))
{
$result = $result . jsonEncode($arr[$i]);
}
else
{
$result = $result . valueTostr($arr[$i]);
}
if ($i != $end)
{
$result = $result . ",";
}
}
$result = $result . "]";
}
else
{
$result = "{";
$i = ;
foreach ($arr as $key => $value)
{
$result = $result . '"' . $key . '":';
if (is_array($value))
{
$result = $result . jsonEncode($value);
}
else
{
$result = $result . valueTostr($value);
}
if ($i != $end)
{
$result = $result . ",";
}
$i++;
}
$result = $result . "}";
}
}
else
{
$result = "[]";
}
}
}
catch (Exception $e)
{
}
return $result;
}
?>
如果使用过程有什么问题，可以给我email.欢迎大家指出错误！